I've gotten a painter app or two for my iPad and iPhone and I'm not real impressed by it.
I'm no painter, but I just don't feel the same kind of control over the medium. Using my stubby fingers on a non-pressure sensitive piece of glass just feels awkward.
And I've tried a Pogo stick, but the one I have is incredibly crappy. So bad that I must have a bad one. But I'm not willing to go buy another one just to see.
My main motivation for purchasing an art program for iPad would be to study the use and manipulation of color, something that I have very little understanding of right now.
Other than that, with the current tools available, I can't see having the patience to WRITE a comic on the iPad, let alone drawing one!
But I'd love to have that change.