I use Adequan for my dressage horse and it made a huge improvement in his quality of life. He's been on it for quite a while now, he's very happy, his movement is so fluid and full of suspension, he's happy doing things like trail riding he used to despise, basically just changed his life, he's a very happy horse.
Downfall, it is expensive, but you will see results. When you start, I think they get it every four days? and then ease it down slowly to once per month, so it is definitely costly. We also ran into the problem where suppliers ran out over the summer, luckily it came back with just missing a month.