While this is a great idea in theory, the cats wanted no part of it.  The issues that I saw with it were:

1)  The entry hole wasn't large enough for both of the cats to fit through (6 1/2 lb one could; 13 lb one couldn't).

2)  It seemed to be much more confining than a normal litter box would be to them.

3)  They definitely disliked the crystals.  Both of them have been using clumping litter for at least 9 years.  

To make it better, I would suggest looking at a more stable bottom piece that can support clumping litter and look at doing a mor open front (say just the bottom lip and then fully open).