Harpagophytum Procumbens (commonly called Devil's Claw) is a tuber vegetable that is used for combatting lower back pain as well as arthritis (both osteoarthritis and rheumatoid arthritis). It appears to have efficacy at this, and may take a few months to reach maximal efficacy (so it would be unlikely something like Aspirin, which acts fairly fast).
Human trials have been conducted on Devil's Claw and arthritis, and although there appear to be a large amount of trials conducted on it many are not sourced online or in Medline (instead mentioned vicariously through systemic reviews and meta-analysis' on the subject) and the power of these trials are limited, due to sample sizes and lack of control for the most part. That being said, at least two well controlled trials note that the efficacy of Devil's Claw is greater than placebo with one saying it is of similar potency to Vioxx (Rofecoxib, a COX2 inhibitor and antiinflammatory).
Not too much research into other effects of Devil's Claw, although one study suggests it has remarkable anticholesterase inhibitory potential with the potency on butrylcholinesterase (a cholinesterase enzyme found in the blood and liver) being greater than Galantamine.
There appears to be much discussion on safety of Devil's Claw, which may have stemmed from traditional users of Devil's Claw warning about excessive usage. In short term trials it appears to be well tolerated, with a lack of long term evidence. It is definitely possible that it may induce uterine contractions when taken orally, and due to this it should not be used by pregnant women.