I resisted ragequitting all the way through to the final room, but after the second time the boss took me out with one hit and I was faced with having to do the whole room a third time (after dying about 50 times just getting the three crystals), I just couldn't go on. Like harmsc12 said, the controls are great and the mechanics are clever, but the difficulty is ramped way too high toward the end for a game with one-hit death and no checkpoints.
if there had been some kind of checkpoint system in the last room (such as the crystals staying destroyed) that would have easily bumped this up to a 3.5, and a little alteration of a few other difficult rooms (like the cheap-death guessing-game room with the five teleporters) would get it even higher. The game is solid mechanically and aesthetically, but having that huge barrier to finishing at the end just blows.