Not too bad, but nothing special. Each world/zone brings about a new gimmick which gets used in increasing difficulty for 5 or so levels before entering the next zone and getting another new gimmick to play with. For the most part the game is pretty easy due to the fact it keeps balancing itself around having to use at least 1 level per zone just to teach the player a new mechanic. Though I didn't try the post-game levels which is where I imagine the true challenge lies.

Kind of concerned how they managed to stretch this into 3 sequels.