Yes, I have had several cats on Cosequin.  My 17-year-old has been on it a few years.  I have had good results.  It is not a miracle, but it seems to help them.  Like the instructions say, it takes several weeks before you see results.  Of course, like with supplements for people, everyone doesn't get results.  I have also talked to one of the veterinarians at the company and was assured of their quality checks, etc.  I have used it for years for my older and/or arthritic cats and dogs

My almost 10 yo overweight male has been on it since January.  He has some joint issues.  It has helped him quite a bit.  Before starting it he would only get up to eat and go to the bathroom. Now he actually comes and hangs out with us and the other cats.  He'll even play again and sometimes doesn't even need his steps to get off our bed.