This is definitely the big issue with iOS music apps. There is a lot of potential there, but unfortunately, doing anything serious is no fun at the moment, because you can't beat having proper hardware to control the real world interfaces apps try to emulate.

I've got a fair few synths for the iPad which I use with a akai 25-key controller, but even with that, knob-twiddling is really fiddly when trying to get sounds.

Would be good to see more apps trying to play to the touch interface's strengths. Figure from Propellerhead is a good example of something with a more thoughtful UI. A good Kaoss-pad like sampler/effect manipulator could also be really cool.