Useful for the weirdly levered.

I tried to help a guy use his computer (you know, for mailing and browsing on the wild side, collecting worms), a few years back, and he never mastered the mouse.

The strange thing was that due to some eye/hand coordination thing, he could look at his hand and things would be fine, but when looking at the screen his arm inexorably lifted so that the mouseball pointlessly hung there. In the end there's tricks for everything, but this device would probably be the better solution.

Not sure his type form a large part of the population, his genes weren't spreading really anyhow.