Maca is mysterious - there's no doubt about that. Researchers are just now beginning to understand Maca and the remarkable effects that it has on both body and mind. The Peruvians for centuries have revered it for its "Fountain of Youth" effect and already its abilites to increase libido, sperm counts and semen volume have been well-documented in both animal and human studies (which I describe in my link on Maca, Testosterone and Libido).
However, Maca's superpowers go well beyond simple reproductive effects. Maca seems to it definitely helps with some of the things that plague us in our modern lifestyles and middle age, including energy, libido, vitality, stress management and so on. And for guys that take care of themselves with diet and exercise as well, it can give you that old I-want-to-rip-her-clothes-off feeling.
So how does Maca work its powerful effects? One would immediately expect it to be a testosterone or leutinizing hormone increaser, but studies have not shown that to be the case. The answer probably lies in its effect on our central nervous system. For example, a 2010 study in vitro, i.e. "test tubes", and on rats found that maca was neuroprotective.  Furthermore, it has (at least in women) been found to overcome SSRI-induced sexual dysfunction.  SSRI's, the largest and most popular class of anti-depressant drugs currently on the market, are famed for their "relaxing and calming" effect, which, unfortunately, can translate to an overly tranquilizing effect on sexual desire. Maca, again at least in women, helps overcome this neurotransmitter soup and reignites passion.
In fact, that is a good word for what Maca does: it reignites passion and desire. Of course, testosterone can do the same thing for us guys, but Maca clearly deserves to be in the same category for its sometimes similar effects. A clue to its testosterone-like potency may lie in a recent study of rats fed a high-sucrose diet where Maca was found to increase the all-important natural super-antioxidants SOD and glutathione.  Increasing these incredibly powerful antioxidants are considered one of the "Holy Grails" of anti-aging research and Maca appears to boost levels of both.
CAUTION: We've had a couple of posters on the Peak Testosterone Forum have pretty negative experiences with Maca. One man said that it gave him E.D. and another lower testosterone levels. Please read this thread for more information. I'm not sure what to think as I have had many positive comments and it is a widely used supplement, but I want to include this information.
In addition, Maca also increased glucose tolerance in these rats and definitely lowered glucose levels in their blood. This means Maca may help with Metabolic Syndrome, which is one of the biggest enemies of our sexual lives and a common source of erectile dysfunction. Maca may also partially do its work by improving some of the basic risk factors that plague modern, industrialized lifestyles. For example, the same study on rats fed high-sucrose diets found that Maca lowered bad cholesterol, total cholesterol and triglycerides. 
I also noticed something very interesting once I started taking Maca: I instantly started gaining muscle.. It is unlikely that Maca did this through increasing testosterone levels, but there is another possible mechanism: Maca has been reported to very significantly lower cortisol levels, at least in animals,  and cortisol is a known mucscle-destroyer.
Of course, all of these impressive qualities need to be better verified in human studies, but the results so far are remarkable by all counts. Maca may be on track to be the next Pycnogenol, which is well-known for lowering multiple heart disease risk factors all the while improving inflammation, blood pressure and erectile strength.
Almost all Maca has the advantage of being organic, so you do not have to worry about some of the issues that plague other plants. It is grown at such a high altitude in the Andean mountains that it has no real competitors and spraying is simply not necessary. I take it in its most natural form from Navitas: Navitas Naturals Organic Raw Maca Powder and I always cook it as that is how the Peruvians have almost always eaten it as well. (For cautions regarding Maca, see the bottom of this page on The Power of Maca.)
Also, some may question why I take Maca, when I normally do not recommend herbs. In my opinion, Maca is in a different category, precisely because it is NOT a herb. It has been eaten for centuries as a food and one authority site comments that the "native populations subsisting on the local diet of the Peruvian highlands typically consume several maca roots a day. The continuous use of this root vegetable as a staple of the Peruvian highland diet for thousands of years suggests a low potential for toxicity". Again, this needs to be proven with a mortality study or two, but considering its widespread use as a food and its long term use as supplement without incident is certainly a powerful testimony.
Of course, do not make any changes without talking to your doc, hopefully a good and knowledgeable one.
1) Annals of the New York Academy of Sciences, Published online Apr 29 2010, "Neuroprotective effects of Lepidium meyenii (Maca)"
2) CNS Neuroscience & Therapeutics, Aug 13 2008, 14(3):182-191, "A Double-Blind, Randomized, Pilot Dose-Finding Study of Maca Root (L. Meyenii) for the Management of SSRI-Induced Sexual Dysfunction"
3) Journal Plant Foods for Human Nutrition (Formerly Qualitas Plantarum), Jun 2007, 62(2), "The Influence of Maca ( Lepidium meyenii ) on Antioxidant Status, Lipid and Glucose Metabolism in Rat"
4) International Journal of Biomedical Science, Feb 15, 2006, 2(1):15-29, "Short and Long-Term Physiological Responses of Male and Female Rats to Two Dietary levels of Pre-Gelatinized Maca (Lepidium Peruvianum Chacon)"