There are an awful lot of claims for the benefit of taking soy supplements, ranging from reducing some of the more irritating side effects of the menopause, through to combating cancer ,osteoporosis and reducing the risk of heart disease.
Most of the clinical data comes from small trials, poorly designed and controlled. The FDA suggest that a diet of food containing soy can be good in reducing cholesterol and therefore reducing the risk of heart disease, but recommend that people derive their intake of soy from foods containing it rather than supplements.
Very few trials have shown any significant negative results, but there are some indications that taking very large quantities of soy regularly over a long period of time could increase the risk of for example breast cancer in women, if taken in very large quantities pre menopause, and the possibility of developing breast tissue in males .
Overall, the consensus seems to be that soy in moderation will do no harm, and may have some good beneficial effects, but that it is better to take soy in foods containing it, rather than just the supplements.