Simple. Difficult.
I died 666 times. A lot of the levels seemed absolutely impossible to get through. The graphics were simple, the color changes completely wrecked my ability to comprehend what was happening, and yet I played through all 15 levels to the end. I didn't swear, I didn't throw a 12 year old hissy fit. For a game to keep me coming back to torture myself, that has to mean something. Either I'm a glutton for pain or this was, in fact, a really fun game.
I'm going with the latter. I loved this game.
P.S. GREATEST MUSIC EVER.