Mostly the smell. If I have to change the whole thing out weekly then it is not really working all that great.
My two like the same box too. Because of the dogs we had to introduce them slowly so they ended up in my youngest's room. Litter box in the closet. So far the clay ones I've tried, claimed one box will last a month with two cats they do not. I managed to stretch it out to 3 weeks but even with daily cleaning it just stinks all the time.
Your babies are adorable. Are they Devon Rex's?