I have used it on Fang for a few months now. My vet recommended using dried cranberry sprinkled on the wet food, which I did for a while but he stopped eating it. Carpon is formulated from cranberries, I think, and it is easier to give than the powder, plus it is the right dosage etc. It does cost more though. The above link will explain it, plus I have emailed Dr. Belfield with questions, and he has always given me detailed answers.
I give Fang two pills a day and he pees in abundance in his box. He does still occasionally pee at the rug spot, not sure if it is the smell, or he wants me to watch, or he is mad at me. He only does it when I am there to see. I know he no longer has crystals because once he peed on a piece of plastic (in front of me) and I had the pleasure of touching his urine to check for them. I just bought him new litter (swheat) because I am thinking he doesn't like the chicken feed, although the other two don't have a problem with it. Fang acts like he is stepping into water, so I am thinking that might be part of our present inappropriate peeing. Anyway, I gave you a lot more info than you asked for. The carpon is great, I highly recommend it.