If you follow the instructions it works well. I have had no major problems, but there some points to consider.
If possible, locate the unit away from the living area of your house as possible. If any solids get left behind when the drying cycle starts, you may as well just leave the house because of the smell. That is my only major complaint, so if you don't have a lot of space, and/or you have more than one cat, you might want to think carefully about buying one of these.
I have two cats, and I'm getting about half of the advertised duration out of the Sanisolution cartridges, about 45 days, and about 2 months out of a box of granules. The granules track everywhere, but it's better having regular cat litter all over. If you have hard water, you'll need to run the cleaning cartridge about twice as often as well. That being said, I still save money over premium clumping cat litter. Doing the math, my break-even point is about 2 years, but that doesn't factor in the additional water and electric costs. That likely negates any monetary savings.
I've used some of the other self cleaning litter boxes on the market and the Cat Genie definitely works better than any other I've tried, with less cleaning and owner attention. It probably won't save you any money, but it will save you some time. So the real question is what is your time (and not scooping poop) worth to you?
A cartridge should last about 2 months for 2 cats ---the advertisement you may be referring to said UP TO 4 months for 1 cat. We recommend a MINIMUM of 1 cleaning per day per cat. (However, best results are achieved when you run it on Cat Activation or if manually, run 2x per day for 1 cat and 3x per day for 2 cats. SaniSolution 120Cartridge in manual provides 120 cleanings / 3x per day = 40 days. Also electricity and water based upon the number of cleanings you do (do to your reported cartridge times) will cost you about $2.00 per month.