I plan to get one someday.
You can do the same thing with a mouse but a tablet gives you a lot more control. Try using your mouse to draw a freehand circle, then you will see what I mean.
The pressure sensitivity is the best thing about it.
I made a thread a while ago about wanting one myself and why, trying to draw hundreds of individual hairs or fur with a mouse is tedious.
You can speed up your workflow dramitically with a tablet.
http://www.graphicsteck.ca/vb/showth...?threadid=1613
BTW
Wacom is the best ones to get.