I use the human equivalent - Revitalife Flexijoint P. Same set-up, I met them when I was an equine Buyer.
I am reasonably certain that the stuff is the only reason I'm still in the saddle, so if I ever had a horse who needed a joint supplement, it's where I would start.
I trust the guy who runs the company to be honest, give reasonable value and be very open to helping you with advice etc. which means a lot in my book!