YES! We've been using it on Taz (14) for a while now. It was the only thing we could find that we could keep her on. She went from limping badly and laying around all day to no limp and frisky again.
Our improvement was pretty quick on Glucosamine, within a week we saw improvement in her (my husband claims glucosamine can't work that fast, but he couldn't argue with the results we saw in her). We tried many other products first and the biggest improvement was in the first couple weeks while we were on Diasiquin. With the Syn-flex, I think we started at 1/2 tsp 2X per day for a week, then went to 1/2 tsp 1X per day. After a couple of months, I went to 1X every day or two and that's where we maintain her now. If we are gone and she doesn't get some for a few days, I give it to her every day for about a week again, then back to every other. She's about 11lbs.