Another advantage is that usually when you hold a mouse, your hand is in pretty much the same position the whole time, whereas with a trackball (particularly th

Another advantage is that usually when you hold a mouse, your hand is in pretty much the same position the whole time, whereas with a trackball (particularly the larger kind), you can roll the ball with whichever of your finger tips, or the long side of your fingers, or even the flat of your hand. I've no doubt that for me, this flexibility leads to much better long-term comfort.