Is the most important molecule for a man testosterone? Well, in spite of the name of this web site, I would probably have to vote for nitric oxide as slightly more important. You can be a little low on testosterone and still do pretty well in the bedroom. But the reverse is not necessarily true. Regardless, nitric oxide can make a much greater improvement in male health and fitness than many men realize.
As a testimony to this, I recently conducted an experiment where I verifiably raised my NO levels and - wow! - did I feel better. This effect will not be particularly obvious for the younger guys that still have ample NO, but for us 40+ males, it can really make a difference. Below I summarize some of the impressive benefits of nitric oxide that have been found:
1. Arteriosclerosis Protection. Nitric oxide, although a free radical, actually protects your arteries from plaque buildup. Researchers have documented several key reasons why this is so, including "preventing smooth muscle cell proliferation and leukocyte adhesion."  Regarding the latter, remember that white blood cells sticking to the walls of your arteries is one of the key processes triggering arteriosclerosis. This is one of the very important reasons that it is so critical that you keep reasonably youthful NO levels.
2. More Erections; Better Erections. Does anyone not know that nitric oxide greatly improves erectile strength? Of course, this is the ultimate goal of Viagra and Cialis. Animal studies shows that nitric oxide improves sexual function in both males and females. 
3. Decreased Premature Ejaculation. Animal studies show that nitric oxide helps with premature ejaculation. Of course, many younger men struggle with premature ejaculation, so this is certainly not the only factor. But it is interesting that this is such a problem as our arterial health steadily declines in the West. One study noted that "the data also suggest that NO inhibits seminal emission, probably by decreasing sympathetic nervous system activity; this may help prevent premature ejaculation." 
4. Improved Mood and Alertness. Now I can't prove this one, but I can tell you that when I Boosted My Nitric Oxide through Food-Based Nitrates, I enjoyed improved mood and alertness. The effect was quite remarkable and reminscent of what I felt when I went on HRT. Now this could be due to many reasons, since the brain is very dependent on blood flow and increase NO will improve blood flow. However, the other reason is that nitric oxide is considered a neurotransmitter in and of itself now. One study pointed out that "NO brings about smooth muscle relaxation by binding to iron in the heme that is part of soluble guanylyl cyclase, stimulating the formation of cGMP. cGMP in turn stimulates a protein kinase, hich phosphorylates the light chain of myosin, eliciting relaxation. The cerebellum contains the highest levels of cGMP in the brain, with its formation stimulated by glutamate acting via N-methyl-o-aspartate (NMDA) receptors." 
5. Decreased Inflammation. Some researchers have stated that "inflammation is the root of all evil." And it sure seems to be true, since it has been implicated in heart disease, diabetes, cancer and many, many autoimmune disease. One would not think that nitric oxide, a free radical, could calm inflammation, but that is indeed the case. One of the nastiest - and I say "nasty" even though you do need some IL-6 - inflammatory cytokines is IL-6. It looks like nitric oxide in many cases lower IL-6:
"NO has an autocrine anti-inflammatory impact on endothelium, owing in part to antagonism of NF-ÎºB activity; since induction of IL-6 is dependent on NF-ÎºB, this may explain recent evidence that NO inhibits macrophage IL-6 production. It is reasonable to postulate that, analogously cerebrovascular NO decreases IL-6 production in the brain. Vascular NO may also have direct neuroprotective activity."  For more information, see my page on How to Reduce Inflammation and Inflammation and Erectile Dysfunction.
6. Superoxide from Nitric Oxide Deficiency. Anything that disrupts natural levels of nitric oxide can lead to a buildup of a nasty free radical called peroxynitrite, which in turn can damage a critical molecule called BH4 (tetrahydrobiopterin). And look at what this study has to say when we interfere with BH4:
"Under conditions in which vascular tissue levels of tetrahydrobiopterin (BH4), a cofactor for NOS, are deficient or lacking, eNOS becomes dysfunctional and produces superoxide rather than NO. Experimental studies in vitro have revealed that NO from eNOS constitutes an anti-atherogenic molecule. A deficiency of eNOS was demonstrated to accelerate atherosclerotic lesion formation in eNOS knockout mice." 
As you can see, this is yet another reason that low NO can lead to tissue damage and arterial plaque.
7. Decreased Refractory Period. Recovery time post-orgasm can be a big issue for many men, even younger ones. Some men actually feel lousy after The Big O and then take several days to recuperate and be ready for another love making session.  For additional information, see my page on How to Improve the Male Refractory Period.
CAUTION: Watch out for elevated cortisol levels. Most of us are under chronic, unrelenting stress and have not learned to manage it effectively. Lack of sleep and overtraining can also raise cortisol levels. All of these, per one study, can sabotage your nitric oxide levels by depleting nitrites. 
CAUTION 2: You can also artificially raise NO by jacking up your blood glucose levels. Unfortunately, this also artificially raises superoxide free radicals (O2-) levels and can lead to vascular damage.  So short term gain for long term pain is not a good trade in my opinion!
CAUTION 3: This is a big subject, but you don't want to "go crazy" with nitric oxide. Too much can be hard on the brain and is associated with a variety of psychological disorders. For those who suffer with migraines, too much too quickly may make things worse. Middle-aged and senior men have already lost about half of their NO, but younger guys should probably not seek every NO Booster they can find and artificially raise their levels in my opinion.
1) Mediators Inflamm, 1997 Feb; 6(1):3â€“21, "Nitric oxide function in atherosclerosis"
3) Neuron, 8:3-11, Jan 1992, "A Novel Neuronal Messenger"
4) Neuropharmacology, Nov 1994, 33(11):1499â€“1504, "The roles of nitric oxide in sexual function of male rats"
5) PNAS, Jul 1994, 91(14), "Nitric oxide mediates sexual behavior in female rats"
6) Medical Hypotheses, Nov 1999, 53(5):369â€“374, "Vascular nitric oxide, sex hormone replacement, and fish oil may help to prevent Alzheimer's disease by suppressing synthesis of acute-phase cytokines"
7) Arteriosclerosis, Thrombosis, and Vascular Biology, 2004, 24:998-1005, "Dysfunction of Endothelial Nitric Oxide Synthase and Atherosclerosis"
8) Clinical and Experimental Pharmacology and Physiology, Nov 1998, 25(11):945â€“946, "THE NITRIC OXIDE SYSTEM AND CORTISOL-INDUCED HYPERTENSION IN HUMANS";
9) Circulation, 1997, 96:25-28, "High Glucose Increases Nitric Oxide Synthase Expression and Superoxide Anion Generation in Human Aortic Endothelial Cells"