I got this so my girlfriend would have something to play on the 360. Now I'm hooked myself.
I thought I wouldn't play it at all, but it grows on you just like a lot of casual games (Tetris, Luxor come to mind). Difficulty modes are well spaced for you to learn from, and I'm sure will add a nice added experience once I'm bored with the lower levels.
I'd like a bigger variety of music style, but I suppose they would find it hard to present dance steps to Slayer. Also, since the game has been out a while in multiple versions, I would have like to see the new Xbox 360 version take advantage of some flashier graphics and a more immersive experience, but that doesn't detract from the fun.
It's also a good way to get the blood flowing in the morning before getting ready for work.
I still stand firm in not ever playing guitar hero!