We have two of the Littermaid Mega LM900 litter boxes, and would buy another one.  They work reasonably well, and save a lot of scooping time & effort.  As other reviewers have noted, it works OK, but not perfect.  Help is needed at times, but it's still a big benefit to have.  The rakes need to be cleaned periodically, since the clumps are not fully dried after 10 minutes.  And the litter needs to be redistributed away from the collection box several times each week as a result of the rakes getting filled with wet clumps.  Our cats used it almost immediately after it was set up, and still seem to enjoy watching it operate.