Actually, I have been using my tablet for some of my pics on dA (including one which was basically a pic I did with just my tablet, which I admit does look a bit derp-ish), though they don't come out nearly as good as the pics I make with my mouse (which is a bit better, though to be honest, not so good as the classic pencil and paper method of art). Yes, using a tablet for art can be rather tricky sometimes, especially when the circles I make with my tablet tend to end up looking like blobs most of the time.
Have you tried making the circle out of a bunch of small short lines and forming it slowly? Patience is a major part of any art, and it sounds like when you're making the line art you try making it in one take, which is never a good idea.
