Had the cat genie for a few months now, have had some issues, Sometimes doesn't scoop all poop out then when it dries it smells the whole house. and the granules doesn't last long. we are going through 1 box a month at least. but it does beat scooping a box. Maybe because I have 4 cats it doesn't do so well.
A box of Washable Granules lasts 5-6 months on average for 1 cat and 4-5 for 2 but will last considerably less with more cats and while it can work for more (as it does for you) we do not recommend it for that many 1-2 no more than 3 per unit.