Used one for a few years and I loved it:
- it's precise, you can really aim small UI elements
- It is paired with actual buttons, index on the nub and thumb on the buttons, so no weird gestures to right click
- It is very quick to switch from keyboard to mouse, your hands nearly don't move
But, there's no mouse wheel or scroll gesture and maintaining pressure on the nub is maybe more tiring than a trackpad.