I have just ordered some of this for my boy so would be interested in any opinions too. However my vet who suggested it says that he has conducted his own unofficial clinical trial on his horses (he breeds, show jumps and team chases and his partner does the same and runs a livery yard so they have a fair few horses between them!) and there was a noticeable difference between cosequin and the other supplements so he only uses cosequin now.
Some people have mentioned that it's cheaper to buy it from US sites and pay import duty and VAT but it sounds very complicated to me!