The app was amongst the featured applications demoed at this year's WWDC. A Youtube Video provided by the developer shows its capabilities.
Ars put the app through its paces and was impressed by the built in functionality beyond basic instrument play:
While the ability to play these instruments would be impressive enough for an application running on a phone, the application also comes with some pretty impressive DAW functionality. You can record, overdub, solo a track, mute tracks, and make basic edits, all on a handheld device. You also have the ability to work on multiple songs, save them independently, erase the last note recorded, erase the last overdub, and erase the entire current instrument track. You can overdub as many times as you want, with the only limitation being the iPhone's processor.
Still despite all this, they note that you are unable to export your creations outside of the iPhone short of connecting to the headphone jack.