Much like doodling on an Etch-a-Sketch, there’s a certain pleasure in overcoming the iPad’s inherent limitations to produce a competent drawing. All iPads thus far have been designed to register relatively imprecise input from your fat fingers. The basic workaround is to draw as zoomed-in as possible, but sometimes it can be tough to draw without seeing the entire picture at once. Your mileage may vary, but unfortunately for me, this rules out the iPad as a sketchbook for quick ideas or gesture sketches.
Incidentally, many of my drawing apps actually seem to be extra laggy on my otherwise zippy new toy, which I hope are just growing pains resulting from the awesome new display resolution.
Art-wise, I think where the iPad really shines is painting; you can’t beat the massive color pallete, and if you paint in large areas of color/tone and work your way into the details, you don’t necessarily have to do a lot of zooming until the very end.
I’m holding out hope for some sort of accessory or software that will tear me away from my real paper sketchbook, but so far Apple doesn’t seem very interested in latching onto my market segment. But all of my artsy e-books look great!