I have been using a PetSafe - ScoopFree Original Self-Cleaning Box for several years and love the product.  I recall the days, albeit not fondly, where I would scoop out any feces every day.  I was skeptical at first that the litter box would smell after a few days but have found that it can last about a month with one normal size cat without smelling.  It is also relatively easy to dispose of the old tray and replace it with a new one. [This review was collected as part of a promotion.]