I tried PetSafe - ScoopFree® Classic Self-Cleaning Litter Box.  The product is working well so far but takes up a lot of space.  The unit moves the clumps to one side of the box and stores there until you clean the unit.  It is also a bit loud when in motion and scares the cats.  The idea is good but if you have cats then you know they can be skittish and opposed to change.  I think I will be going back to the standard box that I clean manually.