I recommend the Litter-Robot 4 constantly. It is $699, it handles my four cats, and it is the best piece of cat equipment I own. And every single time I say that, somebody emails me a version of the same message: cool, Nick, but I do not have $699 for a machine that...
I've owned a Litter-Robot 4 since it replaced the last manual box in this house, and I have not scooped a litter box since. My neighbor runs a Litter-Robot 3 Connect for her two cats. For two months this spring we ran the least scientific and most thorough comparison... 