I still have one of these, but I don't use it anymore. It's actually very comfortable. Your hand sort of drapes over the top, without causing you to bend your wrist at an angle like the Expert Mouse.
I've heard some people criticize the position of the scroll wheel, but I never had any problem with it, other than the fact that it's basically impossible to hold down the left mouse button while scrolling with your thumb.
The problem I have is with the movement of the ball, due to the rubber rollers. When the ball and rollers are clean, there's a lot of resistance, but it's easy to make precise movements. As you use it over the course of a few hours, the resistance lessens, but then it gets kind of jerky. Each roller catches, then releases suddenly, so you find that instead of being able to move in a straight diagonal line, you end up making a sort of stair step motion.
So you clean it all and start over again. I wouldn't be surprised if this behavior is why this trackball wasn't more popular. I know that's why I wouldn't recommend it.
In my fantasy world, though, in which I have the ability to dictate to electronics manufacturers what they produce, I'd have an updated version of this trackball that has a slightly bigger ball, and eschews opto-mechanical rollers for a high speed optical sensor, supporting the ball with bumps, like the current Expert Mouse or the MS Trackball Explorer.
Also, it would be black, because black never goes out of style. Awww, yeah.
