Using music for difficulty is really enjoyable, especially if you use a song that has small lulls or rapidly changing tempos, or just a plainly awesome song to go into a (colourful) battle with. Though using a rather fast song will pretty much result in a dozen lost lives on one song.
I'd prefer a top-down shooter though, but that's just me being used to those games more.
Although I would like a "playlist" mode where you could go against a series of songs.
In all, it's a simple game, but it does well for what it sets out to do.