Oh man, this game. I can't believe how much I keep playing it, even after beating it so many times.
I especially like that there are very few "cheap tricks" in this game, you almost always get a chance to see what's coming. Then, even on the rare occasions something particularly unfair happens, it's usually just one room and there's a checkpoint there. This is how to make a game hard without making it punishing or too frustrating. Not to mention how amazing the music feels with every section, especially (in my opinion) the tower section. That song in there is just perfect.
SUPER GRAVITRON!