If they could prove that it worked, say by a randomised double blind controlled study, it would be all over the marketing.
scienceblogs.com/terrasig/2010/02/18/viagra-horny-goat-weed/
"The active constituent of horny goat weed, icariin, bears no structural similarity to sildenafil ((what Viagra is)).
..
Horny goat weed extract must be used at unbelievably high concentrations – far more than can be achieved in the bloodstream – to cause only 1/10th the effectiveness of sildenafil in a rabbit penile blood vessel model."