I picked up the bamboo ipad stylus to try this very thing out. On the ipad with sketchbook pro and Procreate. My feeling is that it will be an expensive portable sketchbook. It does work, but serves more as a quick sketchy/painting tool, than for anything final. However, you can upload your stuff into dropbox or whatever to continue working on your piece in another computer.
Another option, which I haven't tried yet myself, is using something like this to see your PC's desktop on your iPad. So you could basically work within Photoshop/zBrush/Whatever as you would, only on a portable screen.
The possibility sounds awesome to me personally. If there was a tablet that was a bit bigger than the iPad, and had a pressure sensitive pen, I would definitely try it out.
I would have to say the answer to your question is no. It is possible but it is clumsy and slow, sure you get better with time and effort but why bother when you could be far more productive with a pen and paper.
I usually like doing it on airplane flights and places where a laptop/wacom combo isn't realistic. The lack of pressure stinks. But there are things like
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The possibility sounds awesome to me personally. If there was a tablet that was a bit bigger than the iPad, and had a pressure sensitive pen, I would definitely try it out.
There is a big thread on this already USE THE DAMN SEARCH FUNCTION BELIAS.
http://www.kickstarter.com/projects/jonatherton/jaja-worlds-first-pressure-sensitive-stylus-for-ip
in the pipes. So maybe soon that will be more realistic.