From individual experience, I have used it on multiple older dogs with joint issues from various causes....all have done seriously better on it rather then not. I tend to put them on something new and not change ANY other factor so I can narrow down what works best. Its not magical, and our older dogs still have some stability and muscle loss, but a limp while running will completely disappear while on supplements for example.
Husband who has knee issues from playing baseball catcher and such reports major differences if he takes his glucosamine/msm regularly versus not. He climbs 14'ers while hunting/guiding for months during the summer and fall each year.
So just our personal experience.