TJ, which I purchased for the iPod Touch, revived something I thought was dead in me. I've bought many other music apps since then, but this one remains my favorite. Today I got an iPad. TJ is even more fun there -- much easier to work with chords and wider scales.
Two suggestions occur to me:
1. As currently implemented, pan operates on the whole width of the screen; this is fine until you split the screen. Then, when you play the notes one side, you get left, and when you play the notes on the other side you get right. --> For split screens, pan should be relative to the pad width, not the screen width.
2. Much more important (and, I hope, easier to implement). Shaking an iPad isn't as easy as shaking an iPod. For vibrato and tremolo, could we have the option to substitute x-axis movement? You'd lose pan, of course, but I think it would make the vibrato/tremolo instruments much easier to use on the larger device.