Retro/Grade is a pretty fun rhythm game that's kind of like Guitar Hero in that there are a number of "tracks" you have to monitor, hitting the beat in time to the music. The twist here is that you are playing the game from the end to the beginning because space-time is unravelling, and you have to repeat all the moves that got you to the end to preserve the continuum.
So, yeah, it looks like a space shooter, and it's gorgeous, but it's actually a rhythm game. You have several tracks on the screen you can move your ship to, dodging enemy fire and hitting your own "fire" button with the game prompts you to. Like I said, the idea is to go through the game backwards, copying everything the character had supposedly done before the game started.
The game, like I said, is beautiful, and the music is pretty awesome as well. I did find the game pretty short, however; there are only 10 stages, and after you beat them you can knock around on the "challenge" map, but it's all the same levels with different "challenges" imposed. Some of them are interesting (playing the level again with "disco lighting" is pretty cool), but otherwise it's kind of short. Overall, though, it's pretty fun while it lasts, with an interesting gameplay mechanic that's worth at least trying out.