Unfortunately, textures (both visual and audio) are rough, as are the actual physics, so it seems Payback is more of a "GTA could work on the iPhone" demo rather than an actual game (though of course you can play it, it probably just won't be as fun). Still, we're in favor of any game reaching high on the iPhone, and this one definitely does.
Payback is out in the App Store now, for a substantial $6.99. But it's worth it, especially if you're a big GTA fan, just to see such a complex game on Apple's little handheld. Video on the next page.