I think almost any graphics tablet is better than a mouse for drawing/painting, but the big advantage of sticking with Wacom is the good drivers/software. I had a small cheapo tablet before this one (Genius, I think) and it worked well enough when new, but they didn't provide updated drivers for later OS, so it was useless when I upgraded.

I use a Wacom Intuos3 9x12", I bought the biggest one I could afford, but if I replaced it I probably wouldn't go quite so big next time - it takes up a lot of desk when you need to access the pad and the keyboard simultaneously. A wide format tablet is an advantage if you work on 2 monitors, it means you can access some of your second screen using the tablet. This is handy for applications where your workspace is on one screen but you need to access layer lists etc on the other screen without having to involve the mouse.

Take good care of the drawing surface, scratches will feel very unpleasant under the stylus. And adjust the stylus pressure for a light touch, it'll be easier on your fingers, on the stylus tip and on the drawing surface.