Can CoQ10 improve your sex life? I'm not big on supplements, but the evidence looks strong that it can do just that. In fact, I would throw CoQ10 into the same category as the Super Sexual Herbs that I have written about, i.e. a supplement that seems to help bedroom performance in a lot of subtle but important ways.
Here is a list of 8 ways that CoQ10 can help you when it's showtime:
1. Blood Flow. In patients with a certain heart-related issues, left ventricular dysfunction, CoQ10 improved blood flow.  This has been verified in other patient subgroups as well, including statin-treated diabetics . Most importantly, CoQ10 improved FMD (Flow Mediated Dilation) in men with endothelial dysfunction.  A significant percentage of middle-aged men in modern, industrialized societies have endothelial dysfunction and it is the root cause of most erectile dysfunction. In other words, CoQ10 will very likely help the men who need it most.
2. High Blood Pressure (Hypertension). One of the biggest risk factors for sexual issues in males is high blood pressure. One meta-analysis, which is a study of many studies, found that CoQ10 was significantly reduced high blood pressure.  The abstract specifically states that "we conclude that coenzyme Q10 has the potential in hypertensive patients to lower systolic blood pressure by up to 17 mm Hg and diastolic blood pressure by up to 10 mm Hg without significant side effects." These are impressive results that mean not only will CoQ10 likely give your sex life an immediate erection-boosting increase in blood flow, but it will protect your erections in the long term as well.
NOTE: Remember that most of the evidence for CoQ10 is not in young, healthy patients, but rather with those with issues of aging and a modern lifestyle.
3. Nitric Oxide Protection. One way that CoQ10 does its magic is by protecting your precious nitric oxide from oxidation.  This makes CoQ10 likely a synergistic supplement that will enhance other erectile supplements (and medications), because it helps to preserve and protect nitric oxide through a different metabolic pathway. 
4. Metabolic Syndrome. Another huge risk factor for erectile dysfunction (and a lot of other nasty conditions) is Metabolic Syndrome. In lab animals, CoQ10 improves many of the major risk factors for Metabolic Syndrome.  Furthermore, the improvements were dose-dependent, i.e. the more CoQ10, the less certain key signs of Metabolic Syndrome, specifically inflammation and free radical damage. Even more remarkable was the fact that CoQ10 was able to prevent the typical rise in insulin levels (from insulin resistance) that normally accompanies this condition.
5. Testosterone Protection. Testosterone is the "Sex Hormone," and, in my link on The Causes of Andropause, I discuss how mitochondrial damage is likely one of the root causes of the age-related loss in testosterone that almost all men experience. CoQ10 is known for its mitochondria-protecting superpowers and, therefore, may very well help protect your testosterone over the decades as well.
NOTE: CoQ10 can be expensive in health food stores, but is very reasonably priced on Amazon: Swanson Coq10 100 100 mg Softgels. CoQ10 is poorly absorbed and must be taken with a little fat and ideally a little Vitamin E as well. (Always talk to your doc first and see Caution below.)
6. Libido Protection. Dopamine could be termed the "Sex Neurotransmitter." As I point out in my link on Natural Dopamine Increasers, CoQ10 is also famed for preserving brain dopamine and thus may help with sexual desire and libido.
7. Fertility. Several studies have shown that CoQ10 improves sperm factors and fertility. 
8. Wrinkle Reduction. Topical CoQ10 can also reduce wrinkles if done in the right proportions. There are various Do It Yourself formulations out there, including the Info Packs at Smart Skin Care. What does that have to do with sex? We all know that half of performance is self-confidence and looking better can help dramatically. See my link on Wrinkle Reduction for Men for other ideas as well.
CAUTION: The only caution that I know of with CoQ10 is that one study showed that women with the highest levels of CoQ10 had an increased risk of breast cancer.  This contradicts the work of other researchers, though, who have used CoQ10 to treat cancer. See my link on Alternative Cancer Treatments for more details.
1) Atherosclerosis, 2011 Jun, 216(2):395-401. Epub 2011 Feb 17, "Reversal of mitochondrial dysfunction by coenzyme Q10 supplement improves endothelial function in patients with ischaemic left ventricular systolic dysfunction: a randomized controlled trial"
2) Diabetes Care, 2009 May, 32(5):810-2, Epub 2009 Feb 19, "Coenzyme Q10 improves endothelial dysfunction in statin-treated type 2 diabetic patients"
3) Int J Cardiolm 2005 Feb 28, 98(3):413-9, "Influence of coenzyme Q(10) and cerivastatin on the flow-mediated vasodilation of the brachial artery: results of the ENDOTACT study"
4) J Hum Hypertens. 2007 Apr;21(4):297-306. Epub 2007 Feb 8. "Coenzyme Q10 in the treatment of hypertension: a meta-analysis of the clinical trials"
6) Mitochondrion 2007, 7(suppl 1):S154 S167, "Coenzyme Q10 in cardiovascular disease"
7) J Pharmacol Sci. 2008 Jun;107(2):128-37, "Beneficial effect of coenzyme Q10 on increased oxidative and nitrative stress and inflammation and individual metabolic components developing in a rat model of metabolic syndrome"
8) Fertility and Sterility, Jan 2004, 81(1):93-98, "Coenzyme q10 supplementation in infertile men with idiopathic asthenozoospermia: an open, uncontrolled pilot study"
9) Molecular Aspects of Medicine, 1997, 18(1):213-219, "The effect of coenzyme Q10 on sperm motility and function"