I used to give it to my old horse and found it really useful - I used the sort of granular stuff from Dobson and Horrell as he didn't like the liquid form. It was very good and not too horrifically expensive.
It worked for him - just a little scoop thing full in his dinner, no probs.
I used it on an ex racehorse with dodgy hocks, it really helped him and made him much happier and sounder! It was the liquid form I used. Highly recommend it, and would use it again.
We give it to our cob who has arthritic hocks and it certainly makes him much more comfortable and free moving. We started using one by Gold Label which also contains Yucca and MSM, both being anti-inflammataries, and we saw a benefit quite quickly.
The analgesic properties of devils claw are released when chewed so I would say that the pieces are better to feed than the liquid. You can buy it much cheaper and organically grown from a herb supplier and just weigh out the required amount!