It's better than a mouse for drawing, because it's more like a pencil, complete with an eraser on the other end, but it's the same as a mouse otherwise, except it has pressure sensitivity, which allows for line width (or opacity) variation while drawing (assuming your software recognizes pressure), and it is slightly more cumbersome than a mouse for non-drawing operations.
Better tablets have better pens and more pressure sensitivity levels. Lower quality pens with less pressure sensitivity levels are harder to achieve smooth pressure variation.
It is not like drawing with a pencil though, it is more like drawing with a sharpie, you have to go slow, and it helps to zoom in for smoother lines.
Overall I'd say it's worth it, because it gives you more drawing control than a mouse, but it doesn't do anything that a mouse can't (except pressure sensitivity), so it's not really any easier, except for the way a pen is easier to draw with.
see also:
http://www.plasmaspider.com/viewtopic.p ... 410#p11410