I spent an hour or so noodling around with Garageband this afternoon.
Frankly, I'm amazed that they can pack as much tasty musical functionality into an App that works as good as it does on a Gen 1 iPad. The fact that they sell it for $5 is utterly amazing.
If you have even the slightest musical ear, it certainly is possible to create some quite pleasing tracks. The "smart instruments" let even the totally untrained make some great sounds.
Right now there isn't a lot of documentation for the App. While its not hard to use, anyone who hasn't used this sort of software is, I think, going to have some questions. One thing that isn't addressed at all is the ability to plug a MIDI keyboard into the 30-pin connector using the camera connection kit. This works just fine, but there is no hint of the capability in the "help" sections.
The App seemed very stable. It opens, runs, and save very quickly - with maybe a 3 second delay while it quits to save your unfinished work.
Lets also say that this isn't a full-featured music production program. There aren't nearly the volume of loops and instruments you'll find on the Mac version of Garageband, let alone Ableton or other programs. You don't have the ability to manually edit notes you've played. Crossfades etc. is a little limited.
Apple creating and selling this program for five bucks is, in my opinion, their way of showing off. "Raising the Bar" (in Steve Jobs words) for what other 3rd party developers can do with Apps on a mobile device.
