A couple months back, I picked up Paper Dungeons as part of a gaming bundle in
Groupees' Build a Greenlight 5. You never know what you're going to get, but at prices typically less than $1 per game it's not like you can really go wrong. Truth be told, I haven't even played half of the games that came in that particular bundle, but I did complete Magicians & Looters (I need to review that as well...), and I've played far more Paper Dungeons than is healthy. Is it good, or am I just easily enticed by rouge-like games? Probably a bit of both.
The core concept is pretty straightforward: you run around killing monsters, and each level has a "king" -- a boss with a crown that you have to kill. Defeat him and you pass the level. There are several game modes and five character classes, many of which need to be unlocked by completing levels in the already unlocked modes. When I first started playing the beta (alpha?) version, full access to the Puzzle, Adventure, and Rogue options was unlocked, but the only character I could use was the Warrior. Since then, I believe they've blocked access to some of the other game modes until you reach certain milestones, and they've also added some new features. Let's start with a discussion of the game play, then move to the game modes and characters, and I'll wrap up with some general thoughts on the game.

You start with a blacked out level, with the only areas visible being the squares immediately around your character. As you explorer you remove the black "fog of war" -- and everything you uncover remains permanently visible. Walls block your path but many can be destroyed, damaging you in the process. The interesting thing is that all the fog plays a secondary role: each square you uncover
heals your character, and so the general progression through a level involves you fighting monsters (or smashing down walls) until you have only a few hit points (HP) remaining, and then you go uncover some fog to restore your character to full health. Each time you gain a level, you increase in HP, Mana, Attack, and
Defense, plus you can select one of those four areas to get an
additional boost (I usually go for offense).

Monsters have levels as well, and you typically need to find monsters at your level or lower and kill them, until you level up and can fight the more difficult monsters (though at higher levels you can usually take out something one or two levels above you). There are several archetypes of monster in the game: normal (self explanatory), elemental (stronger than normal monsters but are weak against magic attacks), magic (attacks bypass your armor but they're weak against physical attacks), vampiric (they heal via "drain life" as you fight), poisonous (cast cure, find a temple, or just wait it out without dying), and undead (disease reduces your attack until cured via spell or temple). There are also two "hidden" monsters disguised as either a chest or as a wall, but they're otherwise pretty much "normal" monsters. Besides all of the common types, there are occasional "boss levels" where you have to fight dragons or a demon lord; I haven't seen the latter, but dragons at least are quite tough -- they set your character on fire, so you either need to find a water tile or else you need a much higher level than the dragon to survive.
I mentioned the three core game modes; I still haven't unlocked everything, but the three primary modes are Puzzle, Adventure, and Rogue, with more difficult "Void" versions of Adventure and Rogue available once you've played the game (a lot). The Puzzle campaign has no fog of war (though there are still "healing tiles" as you explore -- they're just not fully black), and everything is predetermined. You do a set amount of damage with an attack and receive a set amount of damage -- there's no dice or other randomness involved -- hence the "puzzle" name. There are 31 levels in the Puzzle campaign, and you can burn through them all in probably 4-6 hours. You can also save the game as many times as you'd like, though there's only one active save slot (at least as far as I can tell). It's a good introduction to the gaming system, so this is where I started.
The Adventure campaign has set maps but adds dice for the fights, and the monsters change each time you play a level, so if you can't pass a level on the first try and it seems "impossible"...well, maybe it was. Restart and you'll get different monsters and monster locations. Otherwise, you just need to find the boss and slay him to finish the level, but since he's the toughest monster you typically need to level up by killing all the other monsters so that you can take him out. There are also limited numbers of save points for each Adventure level (larger levels give you more save points), though this is rarely a problem.

The final game type is Rogue, which is random dungeon maps with
randomized monsters, just as you'd expect, and there are no saves at all
-- one death and you're done. Character progression is kept as you advance through the dungeon, however, so this is the closest you'll get to a real RPG experience. This is the hardest mode, really, but
it also has a nice "one more game" hook. The only issue I have is that
sometimes the game will have problems, and if you're playing Rogue and
you encounter a game-stopping bug (more on this below), you're stuck -- and in some cases you don't even get a high score, which can be really doubly frustrating! But the game is in beta (or alpha) for a reason.

As for the dice, each character can have two sets of dice active for a level (from a selection of 35 total dice sets per class, many of which are "unique" to the class), and you can switch between them any time you're not in a fight. When you first start playing, you're basically confined to one set of dice, but as you progress through the various campaigns, you'll unlock additional (typically "better") sets of dice. The game uses six-sided dice, one with attack modifiers and the other with bonus/spell/effect modifiers. The attack dice will either boost your damage or reduce it, while the effect dice may restore health/mana, increase/decrease your attack/defense, give you extra gold/piety, or grant you a one-time spell (which replaces any current spell that you haven't used).
The one-time spells consist of three class-specific spells (e.g.Shield Rush for Warriors, Power Shot for Archers, Magic Missile for Mages, etc.) as well as five spells that are available to all classes. However, since you only get one spell at the start for a character and the only way to replace it is by having a dice set with the potential to give you a new spell, in practice you'll only have access to two or at most four spells for any character. That creates some strategy as you decide what dice to pack along, but often I find myself using the same two sets of dice for each class, waiting to unlock a better set.
Similar to the dice sets, you have five character classes that you unlock as you play. I believe the Thief and Archer are unlocked in the Puzzle campaign at maps 15 and 31, while the Mage and Priest are unlocked in the Adventure campaign (after maps 15 and 31 again) -- the game will tell you what's required to unlock each character, campaign, and dice set, so it sets goals of a sort. All of the classes have some vague semblance to the standard tropes, but other than the amount of health/mana and attack/defense, they're mostly similar. Warrior, Thief, and Priest are melee classes, so you have to stand next to a monster to attack; Archer and Mage have a ranged attack that you can use to start battle -- two squares for the Archer, one for the Mage -- but once the monster closes to an adjacent square then you're back to melee.

Worth noting is that once you engage in a fight, there's no escape other than death -- yours or the monster's -- unless you happen to have an escape spell. Each class has one spell that's usable in a fight, and they all basically result in the same thing: the fight ends. Of course, unlocking a dice set that gives you access to this spell may take some time, and most of the time retreat is unnecessary. Really the only time I've had to use "escape" (or wish I had access to it) was when I encountered a hidden monster and I wasn't in a position to fight it -- either because my health was low or the monster was too high of a level.
Of the five classes, I'd say the Archer is my favorite, as the ranged attack is often enough to kill a monster before it can damage (or poison/disease) you. You do have lower HP, but you also take less damage from walls, and you can spot "natural" traps (basically poison traps). The Mage is a close second, with more mana but lower HP, with a one square fireball ranged attack. He can spot magical traps (basically one of the three trap types), and he tends to do well after you level him up a bit. Regardless of your class choice, the differences are pretty minor and all seem capable of handling any of the Puzzle or Adventure levels that I've played.

There are other small details, like the temple and shop. Each character can also choose a deity at the temple, which is a one-time choice (e.g. on Rogue) with permanent effects. My one major gripe here is that there's often a lack of information -- e.g. when you first go to the temple, all you're given is the choice of diety -- with no explanation of what the results of that choice will be!You can exit without selecting a deity, then go into the help menu to see what the choices actually mean, but it would be nice to have more information presented at the choice level.
At this point, I've probably already gone into way more depth than needed -- and I have even made a (pretty major) editing pass to trim out some stuff. This is my first stab at this and for a game I've played a lot, so bear with me. Overall, Paper Dungeons is a fun little rogue-like, though the occasional bugs make the Rogue campaign particularly frustrating right now. At some point, you can also access the Void campaigns (higher difficulty versions of the Adventure and Rogue campaigns), but I have not yet managed to do so.
There are two rather large issues right now; the first is the bugs and the second is more with the fundamental game design. Starting with the latter, one big problem with Paper Dungeons -- and many rogue-like games suffer from this -- is ultimately the lack of true variety. Right now there are five character classes and 8-10 monster classes (depending on how you want to count them), which means there's a lot of repetition. There are actually quite a few different monster names, though many end up being the same basic fight but with a different picture and monster description. It's like the character classes: it fundamentally boils down to a choice between melee, melee with a poison attack, or get 1/2 free attacks before melee starts. Fighting a Werebear with a Warrior is 99% the same as fighting a SwampDweller with an Archer, or even facing a high level Djinn with a Mage. There are minor pieces of strategy, but it's all very "samey" feeling.

There's also a lack of core spells and other stats. You basically have health, mana, three spells, three pieces of equipment, and three items -- plus the "special spells" you can get from dice rolls. But since you have to commit to just two sets of dice for a character, you're usually limited to two or three extra spells, and you're fundamentally limited in what you can do each time you play. Every time you level up, what you're really doing is making your character strong enough to take on the next level of monsters, but it all still feels the same -- and your choice of whether to enhance your Attack/Defense/Health/Mana stats doesn't really change things too much.
In the Rogue campaign, you've often purchased all the equipment but the second or third map (or sometimes the first), and you have all three spell books, so then it's just level after level of the same thing. There's no "OMG, check out this awesome new sword/staff/whatever" aspect to the game. Most of the time death at higher levels comes not because of any real difficulty, but because of a mental error -- e.g. accidentally walking into a wall at low health. I personally miss the variety of potions, equipment, etc. that you'd find in the old ASCII Rogue, and while later variants have taken things perhaps too far in complexity, Paper Dungeons almost goes too far in the other direction with not enough variety even for a simple RPG. We'll see if things change as the game continues to receive tweaks.
And the game definitely needs some additional bug fixing. There are times where suddenly you can't move, or you kill the level's boss monster but
the game doesn't register the action. I've found that if you encounter the "can't move" bug, you can
save and exit to the main menu (even in Rogue, you can exit to the main menu and your progress is saved -- which means
you can circumvent the "no saving" aspect somewhat), and then when you
return to the game you can move again. However, if you kill the boss and it doesn't register, you're pretty much done and will have to restart (or in Rogue, commit suicide). Another perplexing bug is what has
my current campaign stopped: I can't attack the boss monster. Ugh.
Level 33, Map 13, and I can go no further -- and not even death is an escape (I tried!), as for whatever reason I end up with a
score of 0. What a rip!
In terms of graphics, there's nothing spectacular on display, with relatively simple 2D graphics, but overall for an indie game the aesthetic works. The Unity engine isn't really being pushed too hard (check out the 3D dice!), but that's fine. Probably the best aspect of the art is the overview maps for the Puzzle and Adventure modes, where you get an overview of your progress and can replay any completed map. The music and sound can be a bit repetitive, and I've turned them off at this point, but again they get the job done for an indie titles.
As for the price, well, I got the game for a couple dollars in a bundle, so I'm quite happy with the purchase. At the regular Steam Greenlight and Desura price of $15, I'd be far less enthusiastic. This is a game being made by one guy, with maybe a helper or two joining for some elements like sound and art. While it caters to a niche market I think the bundle price is closer to what people should pay -- or perhaps $5 at most. There are a lot of other rogue-like games, and while I've played Paper Dungeons a lot (too much, according to my wife), each time I go back I get a little more frustrated. Still, at probably 30+ hours of play now, it's kept me occupied for quite a while.
If you want a somewhat new take on the rogue-like games, Paper Dungeons is worth a look, but I'd wait and try to find it on sale -- or at least try the free demo first. It's not going to win any awards, and we live in a day where there are so many good (and free!) games available that it's hard to stand out from the crowd. As an old school gamer, though, Paper Dungeons resonates enough with me that I keep going back to it every few days -- and inevitably "just a few levels of Rogue" turns into far more than that. Being that the game is still in development, it's also nice to see a few minor tweaks show up on occasion, and if you're really a glutton for punishment there's even a level editor (and user generated levels) to keep you occupied. In short, there are better games, but there are also far worse ways to spend $5 or even $15.
Addendum: This game is a work in progress, as I mentioned earlier, and the latest patch (1.151) adds access to three specific class skills right at the start. These skills recharge (slowly) after use, and they replace the earlier "one-time-spell" that you'd start with. Another change is that levels now prohibit higher level monsters (more than +2 above your level) from being near the starting area. Combined, these tweaks are a nice change and make it a bit easier to manage the earlier parts of the game.