It's a solid game, much better than "Hey Ice King Why'd You Steal Our Garbage?" It play very similar to Diablo, which is a good thing. Combos, charge attacks, blocks, weapons, power ups, and loot are all part of the game. You play as either Finn, Jake, Marceline, or Cinnamon Bun (you can unlock Lumpy Space Princess and Ice King later in the game. Each one has strengths and flaws, for example Marceline can hover over pits to obtain items that Finn cannot, but she has less health than Finn. The dungeons are randomly generated so it's different each time. The plot is that prisoners are escaping Princess Bubblegum's dungeon and you must stop them and find out how they are doing so. Lastly the art style is great, does the show justice and the sprites are great (Marceline's sprite is too adorable).
DISCLAIMER: The only demo there was the X360 version of the game and that is what I played.