From a science standpoint, there is little research on oral joint supplements in horses but what I have seen indicates that they are not useful at all as a preventative. For horses that already have joint problems, research isn't definitive about whether or not supplements help. Little of what is ingested is actually absorbed by the gut - something between 2.5 and like 30%. But, just because it is absorbed doesn't mean it's getting into the joint. Most of the compounds in joint supplements are very large molecules, so they are likely broken down for transport and may or may not be reassembled in the same way.
Research aside, in my experience different horses respond to different supplements in different ways. I think it's trial and error with your particular horse and the particular supplement as to whether it's worth feeding. You've got to feed it, though, for a good while before you can make judgement about whether or not it's doing it's intended function. If you think you see results worth the price of the supplement, then hey, continue. If not, maybe your horse doesn't respond to that supplement. Without research, we have to rely on our anecdotal evidence and gut feeling.
I'm speaking entirely about oral supplements...I can't speak to injections. From comments here, sounds like they are working for many.
As for herbals, I would use with caution. There is no regulation on herbals products marketed for horses in the US, and there is almost no research done with herbals in horses. They can also have drug-like action and can cause interactions with other medications - so if you're looking at herbals, I would consult your vet.