Most supplements are like shooting stars: they look good in the first study or two and then take a slow dive into obscurity. Extracts of icariin, the primary phytochemical in Horny Goat Weed (Epimiedium), are exceptions to the rule: one study after another has come out showing its beneficial properties.
Icariin extracts are also one of the three supplements that I would say gets the most "Wow" comments on the forum and in emails for improving erectile function. Below you'll some of the many reasons why.
I have already categorized Horny Goat Weed as a Super Sexual Herb, something I call a plant that will help you in the bedroom in multiple ways, so this should be no surprise. However, Icariin extracts are actually a little different animal than straight Horny Goat Weed itself, because they have concentrated the Icariin in the Epimedium, creating a much more potent supplement in many ways. (I have received a few comments about Horny Goat Weed tea as well though.)
First of all, here are 10 Icariin Superpowers that make it one very interesting herbal extract:
1. PDE5 Inhibition (w/ Many Less Side Effects). Viagra, Cialis and Levitra are the big 3 blockbuster PDE5 Inhibitors that boost nitric oxide for men with erectile dysfunction. (Actually, these drugs only cause nitric oxide to remain in one's system longer, one of the reasons that they are ineffective in many men.) The problem is that they have a strong crossover into other PDE enzyme systems in the body and can very significantly affect vision, hearing and the stomach. Icariin seems to have much less of a crossover effect and thus a significantly decreased side effect profile. (However, Icariin is a less powerful PDE5 Inhibitor as well.)
NOTE: Icariin has a relatively short half life - about 4 hours I believe - so for maximum short term benefits, one should take it shortly before any anticipated love making sessions. Furthermore, it is about 1/80th as powerful of a PDE5 inhibitor, but about 1/10th at boosting cGMP (which is the real goal). 
2. Dose Dependent Increases in Nitric Oxide. Many studies have shown that Icariin increases cGMP, nitric oxide, etc as mentioned in #1. One nice property, at least according to in vitro studies, is that Icariin increases nitric oxide in a dose dependent manner.  Thus, the more you take, the more blood flow that should be gained and, blood flow after all, is what it is all about.
3. Increasing Testosterone (in Damaged Testes At Least). One study on rats began with researcher injecting cyclophosphamide, a chemotherapy agent that damaged their reproductive system.  Follow-up Icariin treatment actually improved testosterone levels. Admittedly the rats were given a lot of Icariin from a 40% extract, but the testosterone increase was over threefold. CAUTION: Watch out for "testosterone supplements" with Icariin. One young guy experienced fluid retention and erectile dysfunction, even though he had elevated testeosterone and leutinizing hormone (LH), after ingesting ActivaTe Xtreme.  In addition, one animal study showed that Icariin decreased testeosterone and raised estrogen. 
4. Healing Gonad and Bone Mass Tissues. The same study  found Icariin did actual healing on the reproductive organs and helped restore bone mass. Many studies have verified the latter and bone loss, of course, is experienced by many men with low testosterone. See my link Don't Let Your Estrogen Get Too Low for more details.
5. Anti-Cancer For Many Cancers. Preliminary research makes Icariin appear to be a potent cancer fighter in the case of some cancers at least. Researchers on gastic cancer cells commented that "these results indicate that icariin exerts negative effects on tumor cell invasion and migration via the Rac1-dependent VASP pathway and may be a potential anti-cancer drug."  Similar results have been found in lung cells. 
6. TNF Alpha (Inflammation) Reduction. One study on mice showed decreased TNF alpha levels. As mentioned on other pages, TNF alpha is the "root of all evil" and may be significantly responsible for arterial plaque and various cancers. 
7. Cortisol Control and Antidepressant Properties. Icariin's real magic may have to do wtih stress. A number of animal studies have shown that Icariin blunts the typical overly high cortisol rise that results from chronic stress.  Know anyone who struggles with chronic stress? In fact, it is so powerful in this regard that some experts have recommended that it be studied as a possible therapy in depression. 
8. Anitoxidant Protection. Many studies have shown Icariin's strong anitoxidant abilitlites against oxidative damage. One of the things it does is protect against ox
9. Memory Protection. Animal studies strongly indicate that Icarrin can protect against certain kinds of memory loss.  That includes memory loss resulting from aluminum damage.  See my link on Aluminum and the Brain for just how common aluminum exposure is in modern, industrialized societies.
10. Alzheimer's Protection. It looks like Icarrin can protect the brain from the deadly tau proteins that are characteristic of Alzheimer's. 
You'll quickly notice that Icaarin seems to battle all of the standard middle-aged and senior male issues: testosterone reduction, cortisol and TNF alpha increases, loss of nitric oxide/bone mass/cGMP. In addition, it may provide protection against such brain-related orders as memory loss, Alzheimers and depression.
All of these reasons point to Icariin as a legitima All of these reasons point to Icariin as a legitimate male anti-aging supersupplement that has strong anecdotal evidence to support it as well. Furthermore, it seems to be much better tolerated than Viagra or Cialis, making it a viable contender for erectile dysfunction relief as well. As I document in the last half of my book on the Peak Erectile Strength Diet , it can be coupled with other erectile supplements to achieve synergies as well. And since it is extracted from a time-proven and well-used herb, it has a certain safety record associated with it.
It should be noted, though, that the new Icariin extracts are now at 10%, 20 and even beyond. These are obviously at much higher levels than one would get from merely having some Horny Goat Weed tea. Furthermore, any study of Icariin's androgenic versus estrogenic properties will make your head hurt. Some commentary seems to say that Icariin is estrogenic and still others that it an estrogen antagonist. One study says that it dramatically boost testosterone and yet another that it lowers it. The bottom line is that it would be nice if additional research wold be done.
For those interested: Source Naturals, a supplement manufacturer that seems to have a good reputation, has a Horny Goat Weed extract with 10% Icariin: Source Naturals Horny Goat Weed Extract (Epimedium) 1000mg, 60 Tablets .
CAUTION: Don't take any supplement without talking to your doctor first, especially if you are on medications.Â Also, Icariin has occasionally been associated with some of the same symptoms as the PDE5 Inhibitors so long term safety is not really known. In addition, Consumer Lab recently tested four Horny Goat Weed products and found that one of them was contaminated with lead. (The other three did not pass either for a variety of reasons, including poor labelling and low quantity.)
However, my caution has to do primarily with the fact that the new Icariin extracts are now at 10%, 20% and even beyond. These are obviously at much higher levels than one would get from merely having some Horny Goat Weed tea. Essentially, modern supplement manufacturers have created an entirely new animal.nc
1) Asian J Androl, 2006 Sep, 8(5):601-5, "The testosterone mimetic properties of icariin"
2) Clinica Chimica Acta, 9 October 2011, 412:21â€“22, "A novel case of a raised testosterone and LH in a young man"
3) British Journal of Pharmacology, Feb 2010, 159(4):939-949, "Special Issue: Themed Section: Imaging in Pharmacology: Guest Editors: AP Davenport and C Daly"
4) "Study on the estrogen-like effects of Epimedium extractive", YU Yan,YAN Hong,HU Sen-ke,ZHANG Jing-hua(Department of Public Health,Medical5) Eur J Pharmacol, 2010 Jun 10;635(1-3):40-8, "Icariin exterts negative effects on human gastric cancer cell invasion and migration by vasodilator-stimulated phosphoprotein via Rac1 pathway"
6) Zhong Yao Cai. 2000 Sep;23(9):554-6. "Experimental studies of icariin on anticancer mechanism".
7) http://scienceblogs.com/terrasig/2010/02/ horny_goat_weed_viagra.php
8) Biological and Pharmaceutical Bulletin, 2006, 29(12):2399, "Effects of Icariin on Hypothalamic-Pituitary-Adrenal Axis Action and Cytokine Levels in Stressed Sprague-Dawley Rats"
9) Pharmacology Biochemistry and Behavior, Dec 2005, 82(4):686â€“694, "Antidepressant-like effect of icariin and its possible mechanism in mice"
10) Brain Research, 23 February 2011, 1375:59â€“67, "Neuroprotective effects of icariin on corticosterone-induced apoptosis in primary cultured rat hippocampal neurons"
11) Vascular Pharmacology, July 2007, 47(1):18â€“24, "Icariin enhances endothelial nitric-oxide synthase expression on human endothelial cells in vitro"
12) Lab of Integrative Medicine for Lung, Inflammation and Tumor, 2009, 29(4):330-4, "Effects of icariin on inflammation model stimulated by lipopolysaccharide in vitro and in vivo"
13) Brain Research, June 2010, 1334(2):73â€“83, "Neuroprotective effects of icariin on memory impairment and neurochemical deficits in senescence-accelerated mouse prone 8 (SAMP8) mice"
14) Clinical and Experimental Pharmacology and Physiology, Aug 2007, 34(8):792â€“795, "PROTECTIVE EFFECTS OF ICARIIN AGAINST LEARNING AND MEMORY DEFICITS INDUCED BY ALUMINIUM IN RATS"
15) Neuropharmacology, 2010 Nov, 59(6):542-50, "Icariin attenuates ?-amyloid-induced neurotoxicity by inhibition of tau protein hyperphosphorylation in PC12 cells"