Well, I give a few of them.
Babbitty and Spyro have both had recurring problems (Babbitty with some infections, head tilt, etc. and Spyro with chronic upper resp. problems, bordetella, etc.) and with Babbitty, since getting them he hasn't really had issues and the ones he has learned to live with, they haven't gotten worse. With Spyro it's hard to tell because his infections are so deeply set that I don't think they had the strength to fight them on its own, he's currently on the tabs as well as a Chinese herb medicinal mix and we have his problems under control.
With Willow, he often experiences GI problems so he gets the digestive ones. He had been having some episodes and since he has been getting the digestive ones for a few months, he hasn't had an episode.
All of my rabbits receive the joint supplements from Oxbow and of course, since they are more for hopeful prevention rather then solving a problem, I really couldn't tell you if they were actually working or not.
Either way, I think they are mostly preventative so I really don't know if there is a way to tell. Mine just get it in hopes that it will help and keep things from developing rather then actually solving any real problems.