The most mind-shattering game I've seen so far this year, up there with Echochrome...and in a much more visceral way. It's like Rez meets F-zero meets Dr. Mario. The game generates levels from audio tracks and then lets you 'race' through them. No description or video can do it justice. The demo is here, the entire game costs $10 from Steam. I highly, highly recommend it.
It totally depends on how hard you want to make the game for yourself. Any song can be easy or hard depending on the skill level you choose, and what point goals you are trying to accomplish. It really is a very clever game. It's a steal at $10. The thrill for me is in experiencing the drama of a song in a way that allows me to interact with it beyond just listening.