trackballs are probably not a good thing if your used to doing a lot of mouse-intensive work (e.g. gimp/photoshop) and have never used a trackball before, you'll go crackers until you get used to it. For general computing work they are more then adequate and simple to use. Just buy one and try it, see if you like it more then a regular mouse.
I've used a trackball on several occasions and have never found it to be exceptionally comfortable.... The only great thing I can say about it; is if you have no where to mouse over to or need to cover a lot of screen realestate without the desk space to match, a trackball is quite nice.
Personally I prefer touch pads, like those found on laptops.
While it is slightly off topic, you might be interested in
parts of this as they relate to usability and pointers, including trackballs.