Audiosurf is the ultimate time waster if you're on a budget PC. While Audiosurf 2 has its own improvements, the game simply doesn't add enough to warrant the extra processing power needed, or the higher price tag.
The concept of Audiosurf is simple. Ride along a highway designed by the game based off of any song you open, and avoid or collect different colored blocks depending on the mode you select. It's simple fun that can't be beat.