I bought one. The novelty of the "self-cleaning" box wears off after a couple of months of rolling it back and forth. I will admit, however, that it does exactly what it says it will do. The rolling motion sifts the litter through a sifter and deposits the larger pieces or waste into the drawer which you pull out and dispose of.
The drawbacks include the size of it. It looks big, but half of the inside is the sifting mechanism and the drawer. It actually seems to give the cat less room to move around that a regular small lidded litter box.
Wet clumped litter can also clump around the sifter if your cat pees near it. Now THAT'S fun to clean/scrub off.
I actually ended up a couple of months ago taking mine apart and throwing away the sifting part and the drawer, thereby opening up the entire inside of it, and just using it like a regular box and scooping it by hand.
Anya seems to appreciate the extra space, and likes to peek out the extra "window" where the drawer used to be.