I personally have not tried it. I would be wary of all the "natural" stuff out there. Be sure you do some good research. GNC brands seems harmless enough, but it is always better to err on the side caution. I think the FDA does have some warnings about Yohimbe.
Check out:
http://www.personalhealthzone.com/yohimbe.html
Since this is posted in sexuality. I am assuming you are looking for increased sexual energy? There are so many beliefs out there... shark fin, oysters, ginsing, etc... Just be safe no matter what you decide!