Interesting concept for a twin-stick shooter, but imperfect execution. Fun, and easily worth the $2.50.
The enemy spawns and enemy movement change with the music. For example, the big red circles tend to spawn with low beats, and the whites tend to spawn during lulls.
The game supports up to four players, though unfortunately only on the local machine. Multiplayer seems fun, and you can leech points from other players.
Aria has a couple of additional game modes. A "King of the Hill" style mode adds some variety and is great in multiplayer. Free mode takes away the ability for high scores, but allows for tuning all of the world constants. I found this gave quite a bit of replay value.
My only real complaints are that the music response is not always perfect.. and some songs are far harder than others. Other than that, I wished for a bit more variety in enemies.
Overall, very much worth the 200 points for purchase.