This kind of product has several advantages over the mouse. In graphics, release the left mouse button without moving the point is very difficult hand tenses. With the trackball, drop the ball, then release the left click, no movement.
Another advantage: the space required on the desktop is reduced compared to the mouse, your hand well periodically strikes against something.
In some circumstances, I even used resting on my thigh lack of other space.
The only drawback: it is a little more expensive.
Another drawback, if you want: in supermarkets, there is only what is selling well and quickly, mice; this is never suggested. But there is no problem ... at the Amazon.