Cintiqs
I have a the 21UX sibling (2010 model), and I tell you now, if you're an illustrator or you do artwork on computer in any volume, you have no idea what you're missing.
They're not perfect, calibration is a bit annoying sometimes, especially when Wacom release new drivers that muck everything up, but for speed of working, and just getting into the art of it, Cintiqs are awesome. I'll be entombed with mine.
The only downer is software support. They're great with Painter and Photoshop, but Sketchbook Pro (and Design) should be amazing - and they are to a point, but because neither Sketchbook has customisable controls, you can't set up the screen's shortcuts at all, and it they have no Cintiq profiles of their own (brush size rotates the screen, wtf...).
The only thing stopping me from getting a 24HD is the weight and the fact that you can't mount it on an arm (21UX can be, and the standard base allows for rotation...important when drawing on paper). So it should be mentioned that 24HD is more like a draught board...much more suited to anyone working on a board.