My 12 year old lab gets this, and she really enjoys them (not sure how much they help with her arthritis pain anymore though). My little dogs split a Cosequin tablet, same company slightly different makeup. They seem to do well with it, though again I am not sure that it helps so much with their pain anymore.
Mostly they are joint supplements used to help possibly stave off arthritis. Once the pain starts it's less likely to be effective. There actually haven't been any definitive studies showing that glucosamine and chondroitin help with arthritis at all -- just speculation. That being said, my dogs have been on all kinds of supplements for joint support, and they seem to like these the best.