My parents 9 year old GSD mix uses it, but they only give it to her when she is in a lot of pain. (She suffered a broken leg when she was 1 and also tore her ACL, same leg, a year later)
They give her glycosamine/chondortin supplements all the time. Normaly it's the changing of the seasons that cause her the most pain. Like now, it's getting colder and colder, so they will start giving her Rimadyl, to help ease her pain. I have not noticed any side effects from it.