What Garageband on the iPad offers to musicians is quite unique and gives the non musician a way to be creative without ever having to have handled a musical instrument. We could sit here all day debating the evil monopoly and closed source aspects of Apple products, but if said products get people doing creative things, it's win-win.
That being said, it probably won't hold much appeal for the professional and experienced musician, but it will most certainly give a lot of creativity to a lot of people that would have otherwise never considered creating music. Just like Garageband on the Mac, Fruityloops, etc. did.
Just like Deluxe Paint did for the Amiga - guys that couldn't draw a happyface with a pen and paper, it opens up a lot of creative avenues. I was one of those guys that had DPaint entirely rock my world when it came out, lol. I found it completely liberating considering I had no artistic skills to that point, and while I am a musician and probably won't learn much from the iPad/Garageband experience, there's no doubt it will open up a lot of creativeness for the common man.
My 2 bits anyways.