I used to use one. Wasn't too impressed with it, personally. You can't fill it too deep with litter, or the rake won't move. BUT, with only about 2 inches of litter in there, clumping litter tends to stick to the bottom. And you have to use a high quality (= expensive) clumping litter so that the rake doesn't get jammed with goopy litter. In addition, the containers that the waste is swept into don't close, so it doesn't contain odor well. Even when you go to remove the waste receptacle, it's nearly impossible to seal the lid without breaking the receptacle itself! Plus, the receptacles are kind of expensive. Finally, the machines are very hard, maybe impossible, to clean very well. You can't take the whole thing apart, which makes trying to wash the rake (which doesn't come off) and the litter pan without getting the mechanical part wet very tricky. Add in the hefty price tag, and...
But I did use mine for multiple cats. Maybe with one cat, it would work better. Then again, it only takes a couple minutes a day to scoop after one cat, so I'm not sure I'd spend the $100.