Not Very Math
You can really consider Hey Ice King - whose full title I refuse to type more than I have to - as two distinct experiences. If, like me, you're a devoted fan of the Adventure Time cartoon and think that pretty much anything set in that universe is awesome: then this game is mediocre at best. And if you don't have any interest in or knowledge of the Land of Ooo: then it, like the princesses you save, is just garbage.
Credit where it's due, the writing is perfect (not terribly surprising since the script is written by show creator Pendleton Ward himself). The dialog, character art and animations, locations you visit, and even rare voice samples all measure up to the level of quality of the cartoon. This script could be adapted into an episode or two of the show, without missing a beat.
Unfortunately, this is the game's only real victory. The overall design - make your way through the map, fetching items and defeating enemies - is overly simplistic, and poorly served by the levels and mechanics underneath. Navigating the overworld is a pain; inbetween areas of interest, there's nothing to do except walk across the map, and avoid random enemy encounters. These encounters are themselves a complete waste of time, since - although there are stats with levels - you don't gain experience points from fighting, and the only reward for defeating enemies is a random item to slot into your already-claustrophobic backpack.
In levels, the majority of the gameplay consists of beating up enemies, and although Finn and Jake have a handful of moves to use, the main attack's reach and strength make all the other attacks obsolete. There is a healthy variety of enemies, but there are very few distinct attack patterns to avoid, and there's no reason to ever even bother using the attack- and defense-boosting items you'll pick up. Infrequent platforming challenges are made more frustrating than they need to be with lackluster collision detection, shoddy controls (particularly for the 3DS analog stick -- it's too easy to accidentally tilt upward or downward while moving), and a few mechanics which are simply not implemented well enough. The best example is a wind-gust element, which you have to ride or block depending on the context, but which is extremely difficult to see on the screen, and will often blow you into a bottomless pit (forcing you to restart the entire level).
Easily, the single biggest error in the game's design is an incredible propensity for backtracking. Far, far too frequently throughout the adventure, you'll encounter an NPC who requests an item from an earlier region; so you get to go back to a region-gating level, fight through the same enemies again, revisit some other location or level (which may involve yet more re-fighting), then go to the region gate again and fight those enemies even more again. Way too much of the game's running time is simply re-treading ground for the sake of completing fetch-quests.
Ultimately, Hey Ice King's abbreviated length is more merciful than disappointing. I spent a little over four hours on the game from start to finish, and I could imagine a fast-travel system (so as to avoid the aforementioned and abhorrent backtracking) bringing that figure down to three, or maybe even two.
Pen Ward's brilliant writing and the mostly-pleasant art only barely managed to save me from really hating this game. I sincerely hope that WayForward's next Adventure Time installment is better put-together.
Better than: Scribblenauts
Not as good as: Half-Minute Hero: Super Mega Neo Climax
Even if you love Adventure Time: do yourself a favor and watch the game, instead of playing it yourself.