Got one yesterday evening; fricking incredible!!! The morons at the Apple store claimed you'd have to wait and buy Apple branded guitar/keyboard inputs that aren't released yet, but iRig connector for guitar/bass works fine with the GuitarAmp part of GarageBand and if you buy the camera USB connector, you plug one of these $39 M-Audio MIDI=>USB connectors and use it to play all the keyboards included: http://keyboards-midi.musiciansfriend.com/product/M-Audio-USB-Uno-1X1-MIDI-Interface?sku=701379
EDIT=> by plugging into any MIDI keyboard
The guitar amp/stomp box modeling is as good as anything I've ever heard, there are a number of different amp choices, Fender/Marshall/Orange/Vox/Mesa/Line6, make the Amplitude stuff sound like crap....
There are a limited amount of keyboards built in, but they sound unbelievable as well - Hammond B3/M2(with Leslie 122 and rotation speed control/acoustic pianos/Rhodes/Wurlitzer/etc.
Full review later....