I think it depends a lot on the type of work you're doing. If you're donig a lot of interface design and web layouts where you're mostly using things like the marquee tool, the pen tool, and your layer styles, etc. then a tablet isn't really worth the money.
On the other hand, if you're doing digital artwork with the airbrush/paintbrush and freehand drawings, then the tablet is invaluable for the pressure sensetivity, the pen-tilt features (on the higher-end WACOMs) and just the ease of using a pen instead of abrick er-- I mean mouse.
They do take some getting used to, though, so don't get discouraged if you don't pick up on it or see a huge improvement right away.
Through college, I had a 4x5in Graphire. It was a great little tablet and was small enough that I could take it to the lab with me and use it during class. You can do a lot of things with a little tablet, so if you'r enot looking to invest a lot of money, then one of the Graphires might just be the way to go. It's definately worth the money.
Having said that, just over a year ago I upgraded to an Intous2 9x12 and it's amazing. The largert size does tive you a lot more room to maneuver and work.
Graphires are great, inexpensive starting tablets, though.
Whichever size you pick, I do reccommend sticking with a WACOM rather than some other brand. There may well be other good brands out there, but there's a reason that WACOM has become the industry standard. Of the tablets I've tried (about 6 or 7 different brands), WACOM has far out-shined all of them. Other folks here may know of some good brands, but I've never come across one that begins to compare.
Take care,
Kelli