I don't use one, but I also have a big callus on my wrist where it rests on the desk...It looks like I have an lump growing out of my wrist, its pretty funny.
I have a Logitec MX 518 and it works well on the granite im on right now...it even works on my blanket when I'm on my bed!
When I used my family's dell, we had an optical, but it would jump and skip on anything other than the pad. Eventually, it would spaz out even on the pad. It was really annoying to be playing counter-strike and I would be aiming at someone, then suddenly I would be shooting at the wall, or the sky, or my friend...
Mousepads have their usefulness, but not for everyone and every mouse in every situation.