A great place to consult on the efficacy of vitamins, supplements, and herbal remedies is the Mayo Clinic, one of the leading medical establishments in the world. They have reviewed the evidence of thousands of studies and have evaluated various supplements, giving each a grade of A through F based on the hard evidence of it's efficacy in doing what it's promoters say.
Just type in the supplement you are interested in and review their findings:
Saw palmetto (Serenoa repens [Bartram] Small) - MayoClinic.com