Joint Injections

Looking for first hand or second hand experience here... No need to "google it", and post links - I'll be doing that shortly. :)

I had a dressage lesson Saturday with the trainer I got Ransom from. She said he looked off behind, and suggested I get his hocks injected.

We worked on bend, with collection, and concluded there's no point in forcing him to track under, it just makes him look fast & choppy. Instead, she suggested time off, bute to manage pain, and call the vet for injections.

Who has had it done, or seen before & after on a horse that's had it done? Success stories, or a complete disaster? Useful, or useless?

(He's already on SmartFlex Senior, let me go see if I can find the ingredients...)