The iPad doesn't allow extra AUi or AU plugins, only allows for 8 tracks, and is a bit limited in its effects. However, for $4.99, it's steal. It's a lot of fun, and any project done in it can be brought into the desktop version, or even Logic 9. There are a number of things missing from it, namely the following missing:
1. Edit MIDI after recording
2. Edit/Import MIDI for Smart Instruments
3. Import user-made Apple Loops from GB for OS X
4. Manually inputting what chords to use for Smart Instruments
5. Recording overlay (for drums, left hand, then right hand piano)
6. Key change per section
7. Display notes on keyboard/fretboard
But it's a first version. I'm sure it'll get better.