Supplements, especially herbs, scare me. However, there are a few herbs, such as Korean Ginseng and Pycnogenol, that have very, very positive benefits and research results. For example, Ginseng's performance has been so outstanding that one could really qualify it as a food in terms of safety and beneficial properties. Furthermore, and as we'll show below, Ginseng seems tailor-made for the middle-aged and beyond guy battling some aging-onset maladies, including loss of erectile strength, mental sharpness and insulin resistance. It's not an inaccurate characterization to say that Ginseng is a turn-back-the-clock supplement in most of the key areas of health that males are interested in. NOTE: American Ginseng has many excellent properties as well, but we are only talking about the Korean variety here.
Side effects are rare with Ginseng. Really the only major caution is that some supplement manufacturers have allowed lead contamination into their Ginseng products. However, this can be avoided by buying brands, such as Solgar - Korean Ginseng Root Extract, 60 veggie caps , which have been tested for lead and pesticides (and deliver the active ginsenocides specified on the label).
CAUTION: Ginseng does have a few drug interactions, such as with blood thinners, etc., so check with your doctor first as always.
What can Ginseng do for you? Consider these benefits:
1) Erectile Strength and Libido. As I document in my link on Ginseng and Erectile Dysfunction, Ginseng has been proven in many studies to boost your all-important Nitric Oxide and has even been shown to have a solid cure rate (two thirds to three fourths) for guys with erectile dysfunction. In the same link, I also document Ginseng's aphrodisiac qualities.
2) Blood Sugar Control. Early work found that Ginseng had the admirable quality of lowering blood sugar in type II diabetics.  Of course, this was an important benefit of Ginseng considering that such a large percentage of individuals in industrialized societies struggle with blood sugar control. Follow-up research showed that Ginseng lowered blood glucose even in non-diabetics.  Many animal studies have shown Ginseng's protective effects against insulin resistance, diabetes and hyperglycemia.  This is a desireable property indeed, considering the erectile and health-damaging effects of Diabetes and Metabolic Syndrome.
3) Brain, Learning, Cognition and Memory. The list of studies showing Ginseng's benefits to the brain are almost endless. Here is just a partial list of remarkable results such as "better and faster simple reactions and abstract thinking" , "improvement in certain psychomotor functions (i.e., better attention, processing, and auditory reaction time) and social functioning, and mental health" . In addition Ginseng has improved mood and well-being , memory and accuracy , working memory, mood and social relations . One way that Ginseng does its neuronal magic is by boosting the uptake of choline, the memory neurotransmitter that apple juice also increases. 
4) Cancer. Almost all males will get prostate cancer, a common and generally slow growing cancer. One of ginseng's many benefits, according to an in-vitro study, is likely protection against the most aggressive forms.  Other studies show significant protection against stomach and lung cancer  and cancers of all kinds. 
5) Blood Pressure. Ginseng yields a small but significant blood-pressure lowering benefit. 
These are the major advantages that most guys are particular interested in. However, it is important to note that Ginseng has also been found to benefit the central nervous system and battle certain neurodegenerative diseases and has excellent anti-inflammatory and antioxidant properties as well. Again, as with Pycnogenol, the list of Ginseng's benefits is difficult to summarize in one short page.
1) 1) Diabetes Care October, 1995, 18(10):1373-1375, "Ginseng therapy in non-insulin-dependent diabetic patients"
2) Arch Intern Med, 2000, 160:1009-101, "American Ginseng (Panax quinquefolius L) Reduces Postprandial Glycemia in Nondiabetic Subjects and Subjects With Type 2 Diabetes Mellitus"
3) Curr Ther Res Clin Exp, 1996, 57:959 68, "A double-masked study of the effects of ginseng on cognitive functions"
4) J Ethnopharmacol, 1986;16:15 22, A double-blind, placebo-controlled clinical study on the effect of a standardized ginseng extract on psychomotor performance in healthy volunteers"
5) Int J Clin Pharmacol Res, 1999, 19:89 99, "Effects of a standardized ginseng extract on quality of life and physiological parameters in symptomatic postmenopausal women: a double-blind, placebo-controlled trial. Swedish Alternative Medicine Group"
6) Physiology & Behavior, Apr 2002, 75:739-751, "Modulation of cognition and mood following administration of single doses of Ginkgo biloba, ginseng, and a ginkgo/ginseng combination to healthy young adults"
7) Physiology & Behavior, Apr 2002, 75:739-751, "Modulation of cognition and mood following administration of single doses of Ginkgo biloba, ginseng, and a ginkgo/ginseng combination to healthy young adults"
8) Neurochemistry International, Jul 1992, 21(1):1-5, "Actions of ginsenoside Rb1 on choline uptake in central cholinergic nerve endings"
9) Horm Metab Res, 2005, 37(3): 146-151, "Improvement of Insulin Resistance by Panax Ginseng in Fructose-rich Chow-fed Rats"
10) ARCHIVES OF PHARMACAL RESEARCH, 27(7):790-796, "Wild ginseng prevents the onset of high-fat diet induced hyperglycemia and obesity in icr mice"
11) Acta Pharmacologica Sinica, Sep 2005, 26(9):1104-1110, "Antihyperglycemic effects of total ginsenosides from leaves and stem of Panax ginseng"
12) Life Sciences, Aug 8 2000, 67(11):1297-1306, "Anti-proliferative effect of ginseng saponins on human prostate cancer cell line"
13) Int. J. Epidemiol, 1998, 27 (3): 359-364"Non-organ specific cancer prevention of ginseng: a prospective study in Korea"
14) Int. J. Epidemiol, 1990, 19(4):871-876, "A Case-Control Study of Ginseng Intake and Cancer"
15) Annals of Pharmacotherapy, Published Online, 6 December 2005, "Systematic Review of the Effects of Ginseng on Cardiovascular Risk Factors"