i have used devils claw on several horses over the years and have found it very usefull. it does take a while to show an effect as it is more of an anti inflammatory than a pain killer. if you use a product such as no-bute or bute free. they combine it with other things and it is more effective.
boswellia is better for long term use - same effects but less rough on the stomach. It took about a fortnight for me to see a difference with DC. I soak the DC root in hot water and then chuck the whole lot in the feed as the infusion gets into the blood stream faster.
Liquids are usually the most effective form of medicating. We use devils claw in Bute Free, which also contains MSM and Yucca, it is very effective. I think it took about two weeks to be fully effective.
About two weeks and the result has been phenomenal, very different neddy who wants to go forward so much he is cantering most of the time. This is a huge change from a very unhappy boy who refused to go forward at all!