i used it on my mare a few years ago. she had a bit of a stiff shoulder and it seemed to help, however this was an on/off thing so not able to really tell if it was the devils claw or she just didnt have any stiffness at that time.
Do your research first! There are a lot of cheaper DC supps out there which don't have an awful lot in them.
I used Riaflex Devils Alternative for a while, and it definitely made a difference. Unfortunately, Alf decided he didn't like the smell, and now spurns any feed that I have tried to conceal it in. Hie fieldmate, who can be very fussy about supplements, hoovered it - so I guess it just depends on the individual horse as to whether they'll eat it or not.
I found DC one of the better ones (and I've tried loads on my old arthritic WB) and its not too expensive. Think its definitely "suck it and see" with them all as what works for one horse doesn't for another. Best of luck!
Definitely has a significant effect. If medication rules aren't an issue I would def. go for a try! You can also nose into homeopathic treatment plans e.g. I've had good success using Zeel or Osteel in conjunction (Tablets). Not as invasive and some horses seem to not take to the DC too well stomach-wise.