Wacom Intuos 3
I have had a WI3 for almost two years. I think they make so many tasks easier than using a mouse. Whether you are editing photos, drawing paths, illustrating, they are brilliant.
That said, I would never do away with the mouse altogether. I still use the mouse for general use and swap to the pen as needed.
As the new Intuos 4 tablets are out, you should be able to pick up the I3s cheaper. And it helps if you are a student (or have a relation or friend who is), as many places offer discount. (Bless my Fiancée and her Masters).
As for brands, I only really have experience with Wacom. I have seen cheap tablets around, but wouldn't recommend any. You get what you pay for, as with most things. If you shell out 50 quid for a tablet, chances are it will be pretty useless and won't help much (and will probably stay in its box).