Desktop Dungeons has sucked away all of my free time this evening. It's kind of like a rogue-like meets minesweeper kind of game, where the goal is to beat the boss of the randomly generated level by any means necessary (usually meaning leveling up alot til your capable of killing it). There aren't persistent characters, and games are very short. However, there is persistence in unlocking classes/monsters/items, and I think the game does these things quite well.
It's certainly hard, and my failure rate is probably around 75%, but since games last 5-10 minutes that's not a big deal. Every once in a while the random map generator will screw you over, but generally only in the beginning of the game which leads to an easy instant restart. Otherwise I think it's a brilliant way to waste some time.
The thief is pretty awesome - there's no real attempt at class balance as far as I can tell. Although maybe that changes once you unlock the "real" (i.e. rated) dungeon; I haven't done that yet. I was utterly addicted to this for, like, three days and then the novelty wore off on me. Absolutely worth the download though; I see myself killing time in it in 15-minute increments for years to come.
Yeah, this game was insanely addicting while I was unlocking everything, but once that was done with I didn't really care to beat the 'advanced dungeons' with every class. Desktop Dungeons v2 could be total crack if they just give it a biiiit more complexity/variety in gameplay.