I won a container of Cosequin ASU this summer. Mare has been on Legend monthly injections and SmartPak's SmartFlex Senior Herb-free for a year or so previous to starting the Cosequin ASU (I would have NEVER bought it had I not won it!) but I noticed quite a big difference (I had a big difference with Legend, and no difference with the smartflex). One of the things I've noticed in addition to her feeling overall better undersaddle is she is not stocking up (she would stock up whenever she was stalled before).
I don't have personal experience with the other Cosequin products, but I am now shelling out the $$ to keep maresie on her ASU
I think that many times people think Cosequin can be a replacement to joint injectables (like Legend or Adequan) and are disappointed with the outcome. If you were only interested in a glucosamine supplement I would tend towards the IV glucosamine injectables or the IM injectables over an oral supplement.