My mare is twenty four and has been on Yucca, MSM, Corta-Flex and Apple Cider Vinegar for years. If for some reason she doesn't get this for a week (I'm ill, on vacation whatever) She gets very stiff in the front end and lightly limps on one of her front feet. It takes a good three to four weeks of this fed daily to get her back to sound and smooth again. I guess that said, I know its working.
If you're looking for cheaper try the Yucca ($10 a month) MSM ($10 a month) and Apple cider vinegar together, see if this helps. The corta-flex is the only costly supplement of these four.
Edit to add: I feed 1/2 cup of the apple cider vinegar daily.