Astropad is pretty nice, I occasionally use it on my iPad3. Unless they seriously improved the latency over both wifi and lightning cable it's not as fun as it could be.. It has nasty artefacts/mpg-like compression appearing when things move, which obviously doesn't happen with a cintiq. If it did work seamlessly as a cintiq replacement I'll likely sell my cintiq.
Hopefully the display quality will force Wacom into upgrading their smaller graphics tablets to compete, too.