Beat Hazard Ultra
I tried Beat Hazard way back before it was Ultra, but couldn't get into it -- the Ultra expansion/DLC really adds a lot to the moment-to-moment gameplay variety, such that it can stay exciting over lengthy songs. (Incidentally, I also have a much more interesting playlist to use, now.)
Beat Hazard's action-heavy gameplay makes it a lot more engaging than other schlocky based-on-your-music games, and the musical factor also gives it an edge over a largely-stale space shooter genre. The explosive visual presentation doesn't hurt, either ... except when the screen gets too busy to see what's happening.
With well-paced enemy variety and your own favorite music, Beat Hazard (Ultra) can be great fun when you've got some time to kill.
Better than: Audiosurf
Not as good as: well... a real video game.
Also relevant: it's, what, $6.50?
Progress: Sub-Commander rank