These are very preliminary thoughts about this app and I haven't really played with it enough to uncover its full potential.
I bought this yesterday and it is a very impressive piece of programming and human interface. The ability to generate musical instrument tracks purely by singing into a microphone works very well.
I was correct though that you really need to be able to sing on key. The auto pitch certainly helps but If you can't sing on key for all the parts required, then your parts will be all over the place in terms of pitch. There's additional auto pitch controls here but I need to play with them more to really make this work as needed.
You also need to think ahead of the structure of your song since you are basically singing each part in real time. Having a song structure or chord chart in front of you might be very handy when laying down the individual tracks.
There are also more instruments then the demo shows. The video demo shows the power chords guitar, assorted drums and bass guitar, but there's also a lead guitar, saxaphone two synths and an organ patch.
There's full control over level and stereo placement of each instrument, which I believe needs to be set before recording a part. The metronome also has level and tempo controls.
You can also save and load and save individual parts, as well as email a recording for export.
FYI, I am using an iPod Touch 2G with a mic/headphones combo and it works very well, with no latency at all. You can sing along with any music already on your iPod.
All in all, this is a very impressive app and it seems to have enormous potential as a song writing tool.
Voice Band at the app store: