Some tablet PCs might be ok, depending on what sort of tablet screen it has, but the one I purchased had terrible pressure response and accuracy. In practice it felt like drawing with an old disposable Bic.

I now use an Intuos3 6x11", which I picked up for about $300. Excellent pressure response, no perceptible position jitter, and just a much better feel in general. I sometimes miss being able to turn the computer to get a better angle, but there are quick rotation controls in many drawing apps.