I've used Liquid Next Level and Vet Value (I think its called that....) on two horses.
On one of the horses I feed it to it just helps him track up a little better and give him a little more length in his stride. The other horse has drastic changes. Without it, she is hard to push out in stride, she drags her way along and pretty much plods along. On it she is active with her strides, over tracks, and it is easy to get her into a nice stride. Though I have seen many joint supplements that don't work, and I know with the mare, she actually gets lame/stiff off of some of the joint supplements.