I have used this trackball for over two years and it has completely replaced my old mouse. I have used it in both windows and linux, and all the potential uses. In common desktop applications (web browsing, file management, etc.) there is no difference in function compared to a mouse. To the game works fine, but it's definitely one of habit.
The main advantage is that it requires less hand and arm movements than a mouse, which is not only more comfortable, but I also think it helps to prevent RSI. It also takes up less space on your desktop, and it is always in the same place, so you can reach for it without groping (if you do not take your eyes off the screen).
Do not use the keyboard, you can sit back with the trackball on your lap or on the arm, a great way to change your working position. :)
The main disadvantage is that it lacks the middle button / scroll wheel. The scroll function is not the problem, there are two small buttons that can be used to scroll up and down. However, many applications use the mouse wheel as a middle button, and that I lack.
It needs to be cleaned often to the ball will roll good. Cleaning is easy, the ball is just to lift out so that you can wipe away the dust inside with paper towels. Clean to the ball with detergent becomes dry and rolls poorly for a while until it got a layer of grease from your fingers.
Sometimes, when the trackball is stationary, the pointer slowly begins to drift across the screen. Stir one then the ball will stop pointer. There is perhaps a certain position on the ball that can not be read by the sensor? Either way, this happens quite often and is not a major annoyance. It also happened when I had an optical mouse.