GarageBand for iPad Review | Music | iOS Central | Macworld
For $5, I can't see how you'd go wrong.
So I got it. It's a fucking hoot to play with, easily the best audio app on the iPad. This should serve as a wake-up call to all the other applications out there. The amp models sound great, the piano, bass, drums, and synths all sound good, and the recording and editing is simple and intuitive.
On the Apple boards, someone's already connected a Roland kit and a MIDI controller to it via the camera connection kit, and it does indeed work. Pretty amazing, especially for $5.
I highly recommend it.