I've been using a Wacom Intuos tablet for years, and really can't live without it at this point. I've always have wanted to try the Wacom Cintiq, but at the price point it hasn't seemed worth the investment.
I am, however, very interested in Astropad, which will "turn your iPad into a professional graphics tablet for your Mac." The app itself is $49.99, but looks like you can try it for free.
If it works as well as the demo videos, I think I'd be more than happy to spend $50 on this.