I use Litter Mate combined with clay litter. Litter Mate causes clay litter to clump and in my opinion, it clumps better than every brand of clumping litter out there (and I've tried them all). It also makes the litter nearly odor free. With 12 cats here, it is terribly important to control odors in my house.
First investment is a bit high as you have to fill each box to at least 3 inches to allow it to work right. But after that, you scoop out the waste, then only top off the box to keep the 3 inch level. You never change it out, only need to pour one box into another to clean the box occasionally (if they aim high when the pee). You would need to replace it only if you have a parasite infection that gets into the litter.
I tried this on a whim, and haven't fully replaced the litter in my house for a year, yes I said a year. I replace boxes on a regular basis and unless the cats hit the plastic, the used boxes don't even have an odor to it. Yup, sounds to good to be true, but again, with 12 cats, I think this is the best product ever.
I did the math of the cost of this. After the initial investment, your cost goes way down. I used to use about 40 pounds of clumping a week and now go thru a 40 pound bag of clay in about a month, plus the Litter Mate. I'm saving close to $1000 a year on litter. You have to order it online, or if you are in the midwest, the vendor goes to cat shows and sells it there. His daughter just started to work for him and she is going to try to get the product further out such as areas like Denver.
Here's the link if anyone is interested: http://www.littermate.com/