Astropad is a good fit for me. I already own an iPad; and for <$100 I get Astropad and a pressure sensitive pen. This means the draw-on-screen experience of the $1K Cintiq and no extra peripheral to dig out.
I saw on other sites where someone complained that "pro" tablets give 2048 levels of pressure sensitivity while ipad pens give only 1024, but I can't tell a difference. (Esp when the LCD only has 256 levels per color channel!)
The geeky side of me is more impressed with how they got such low latency and high fidelity over the same wifi pipe used by other screen-mapping apps (Duet).