Just two add two pennies some time later --
I used to have the huge MS IntelliMouse Explorer; it was my favourite ever rodent until it died from overuse after a few years (it really looks a mess). Maybe it helped that I have big hands, it fit perfectly.
After it died, I bought the new IntelliMouse Explorer (as the old one wasn't available anymore), which is a lot smaller and has smaller buttons. After using it for some time though, I started to develop serious pains in my wrist, so I gave it to someone else who also got the same problems with it, which is a shame. If I could get my hands on one of the old, huge MS IntelliMouse explorers, I'd be well happy.
At the moment I use an MS Trackball Explorer, which does the job nicely. Although I really don't like the way the buttons are placed, it works well, and after you get used to using a trackball it becomes just as intuitive as a mouse. I have a finger trackball, not a thumb one. I prefer it to a mouse when using graphics programs such as photoshop, the gimp or blender, but I don't find it more accurate than a mouse (which I had hoped for). At the same token, I don't find the mouse more accurate now either.
As an avid FPS player, I've been trying to get used to the trackball. It's like learning to walk again, you have to have the patience to learn to play a game you used to be really good at. I'm improving by a lot, and I'm almost up to par with how I play with the mouse.
As it is, I'm waiting for Logitech to bring their MX series mice out with laser support (and corded, I hate using batteries), which I will probably use for FPS (I'll keep using the trackball for all other games and uses).
Also, I believe you got your pro's and con's mixed up