Use TouchOSC.
Extremely easy and intuitive to use, though it will take a little bit of busy work to set up. We're planning on switching over the touch screen we currently use at the studio to two iPads running this (we've tested this with the iPad we currently have and it passes with flying colors). Because it connects directly with your computer via wifi, it's also very practical for live use (I imagine that's what you're thinking of using it for).
You can add pretty much anything, knobs, faders, buttons, etc., and then assign whatever midi message you want to come out, (program change, CC data, etc.).
I can't recommend it enough.
Here's a little bit more information about OSC: http://opensoundcontrol.org/introduction-osc (a little bit techy).