The brilliance of Garageband on the iPad is this: the app seems to have been designed specifically for people who know just enough about constructing music to be dangerous.
All of the instruments have "smart" options, which is to say that the user has the option to let the instrument be the smart one, in case the user's musical smarts are lacking. You can play a traditional keyboard or use a guitar fretboard via the touch-screen, but you can also select versions of the keyboards that are divided up by chord: C, F, G, A-minor, etc. So I can start a bass track by playing two bars on the G chord notes, one each on the C and F chord notes, and return to the G chord notes to finish up. Then I can go to the keyboard and have it do a G arpeggio, a run of C and F notes, and another G arpeggio. Then I can chunk in some rhythm guitar using the same chords, and noodle around on another guitar until I find a lead melody that fits with my backing tracks. When I have one eight-bar section done, I can move on and create another one, copying the bass line forward if I want to reuse it, and looping repeating elements so that I don't have to play them over and over again.
Most importantly, I can do all of this at a tempo of 60 beats per minute, and when I'm done I can dial it up to 130 bpm. The result is a fast-paced song with no dissonance, nothing off-key, and a whole lot of musical density, without even that much investment of time.
What I need now are some apps that let me do this with the rest of my life: block it into manageable chunks, make sure the chunks line up without clashing or conflicting, and run the whole thing at a speed that's easy to deal with.
Any day now, I'll be a virtuoso!