It depends on what you're looking for in a pointing device. A trackball only requires the space of the trackball itself, whereas a mouse requires space around it. If you're an AutoCAD user then you can pan indefinitely with a trackball, whereas with a mouse you run to the edge of the mouse mat and then have to lift it up and start over. Some people claim RSI is less with a trackball, but to be honest if you're overdoing things enough that a [good] mouse is causing you problems, a trackball won't help one bit.
There are a few annoyances though. Although modern trackballs are optical, you still have a moving ball, and I find that it gets gunked up with the same crud that gunked up traditional mice.
There are thumb operated and finger operated trackballs. I have a finger operated and there is a slight problem for using it with AutoCAD. Because you're using your finger to move the cursor, you cannot easily press the middle button (or scroll wheel) at the same time. I find that this slows me down considerably compared to a mouse. A thumb operated trackball is probably better for this.