After spending a semester with ProTools creating multi-track recordings, I am going to have a lot of fun with this.
I'm not musical to any extent and I don't even own an iPhone (N1 here), but I'd love to play with this app after watching this tutorial. I will be recommending the app to a few of my musical friends.
I own the app. It's pretty fun with a couple people contributing, but with more than 3 the little iPhone speaker gets drowned out by the laughter.