" @JJWeatherman: I think that the appeal of Zombies is its humor, so I wouldn't really classify it as a music focused game. "I dunno man. I will play that game randomly just to hear that song. I couldn't care less about killing zombies. Plus, the zombies come out of the ground in sync with the music. The music reflects what's happening at any given point. Sure, it's not interactive music like Rez or Groov for example, but it's still a very important part of the experience.
There's something about audio focused games that makes me super interested. Rez, Audiosurf, and Beat Hazard are some of my favorites. I think I'd like Groov. It really does seem like a steal for only 80 MS points.
(Oh and I would totally count the "I made a game with zombies in it" game as a music game too. The music is the main draw, for me anyways. It's only a dollar too!)