It fits *so* comfortably in the hand, especially for someone with long fingers like me. (Putting my fingers on a top-ball model means my thumb is too short to reach the buttons!)
It's simple to clean, has never given me a moment's grief, and is still going strong today. Considering it's been disconnected (*why?!?!?), I'm thinking about getting another now just as protection against the day it finally dies, I don't want to ever use anything else!
My complaint is that the mouse software screws everything up. It is simply buggy and has wasted my time on every system I have tried to install it on. It really screws up NT (Blue Screen).
If you can get your hands on one of these, do it. Logitech no longer makes a comfortable trackball with the ball on the left side.