3 months ago I risked and after a lot of research I bought a Motion Graphics LE1700. In case you dont know what it is, imagine a Ipad that runs Windows XP with a Wacom screen.
So i installed Zbrush, Photoshop CS4 and PaintTool Sai and been using it and have to say it works amazing. From life drawing to zbrush sketching.... in the park, in bed, in the couch with girlfriend, on a trip....
2 batteries, each 3 hours power, a "View Anywhere" screen for different angles... It changed the way I do art for sure.
For the non believers, I work in videogames, I have a cintiq 21x at work and a Intuos 3 A4 at home so I am aware of the different levels of pressure and how important they are.
In Zbrush I can get away with it and PaintTool sai is like a super sketchbook pro so it compensates.
Finally shortcuts are a bit of pain since you only have side buttons, but its enough, you quickly get used to it. And being able to carry it ANYWHERE, just no words for it.
Some work done with it:
Paint Tool Sai