I am a big fan of this game. I originally got it before official released and loved it then, and love it even more now.

The only way I can explain this game is it is almost like a very fast pace 4x game.

You explore the dungeon, finding rooms, loot, and other characters
You expand by claiming rooms as your own using dust to power them. (And surprising choices on what you claim and don't claim can change you from winning to utter defeat)
You exploit by taking a whole wing of the dungeon and build up all your building there and stockpiling resources for the next floor.
You exterminate, because if you aren't opening every door and killing everything that moves, you will not be leveled, you won't have tech and you won't have resources to make it through.

Another thing I love about this game is you will never stop being surprised. I am 60 hours of game play in, I just had a combination of characters in my team that caused a major event to happen between floors to the point I am standing up walking around the room like, crap, crap, crap, now what am I going to do. This never happened before, why why did this happen to be 2 floors before the end aaahhhhh....

Pro:
-Quick action
You can load up and play through a floor in 10 minutes, or play through a game in under 2 hours (If you live 2 hours :) )
-RPG Elements
Your characters don't really get experience, but you can level them with food. There are a lot of characters each with unique skill sets that are all kinda fun. If you are like me, you even end up having teams you like. Ultimately, the only thing you must have in you party is a fast hero, and there are plenty of them.
-Building Elements
You might have to unlock them all, but there are choices for every play style.
-Replay Value
Even with just the variate of characters the game has massive replay. Then, they added different pods before release which add a whole new level to the game by changing the core rules.

Cons:
-Difficult
It is just unforgiving. One mistake and your game can just be done. Two mistakes and you are a better man then me if you survive. (Many people will call this a pro)
-Monster and item variety
There just isn't a lot of options for monsters and loot. You get better version of the same gear and there is some variety in monsters, but I feel it is a bit lacking.
-Cost
How can this company stay in business and make me more totally awesome games if they only charge $12. They should raise it to $30 ;)