I've been giving Angel Cosequin DS since August. She had a suspected tear of her ACL, though the vet wasn't completely sure. I decided to wait on xrays and other procedures (surgery) and go the supplement route. If it was a full tear, I knew I wouldn't see good results and I could proceed. She's made amazing progress being on this supplement. She used to hold her leg up and not use it much, but now she has a barely perceptible limp. Some days she doesn't limp at all. She's been on this plateau for a month or so. I'm going to start some gentle exercise with her again, to see if building up her leg strength helps her over this last hump.
I'm very interested in someone mentioning that liquid K9 Level 5000 supplement. I'm going to read up more on that.
But in short, to answer the original poster's question, I am very happy with glucosamine/condroitin supplement and I would recommend it. I can't stay if it will help prevent issues, but it sure has helped after an issue developed.