I have tendonitis in my thumb from years of playing in a rock band that requires fast pumping 8th notes all night. When I saw this thing, I thought maybe it will help, as when I play a 5 string, and I'm primarily using the 4th string, I use the 5th string as a thumb rest and it seems to give my thumb a break.
I've only had the thumb fin a couple of weeks but so far so good! I found it very comfortable on the past few shows. It'll probably take a few more months to know whether it helps sort out my tendonitis.