Will Maca (Lepidium meyenii or L. peruvianum) improve erectile dysfunction? The answer is 'yes, indirectly'. This is one of the few herbs that has some decent evidence for being a good, old fashioned and legitimate aphrodisiac. This is a plant that grows in the high altitudes of the Andes in Peru and has been eaten by the natives for centuries for both food and increasing sexual potency. And, in this case, it looks like the natives knew what they were doing: studies on rats and mice have shown it dramatically improves sexual behaviors.  In other words, at least in rodents, science has shown that Maca is a powerful aphrodisiac.
What about in humans? Well, even though no studies have been done on Maca's libido-enhancing powers to date on humans, Maca has been shown to also be something sent down by the fertility gods. Researchers examining nine men, aged 24-44, found that "the semen volume, total sperm count, motile sperm count, and sperm motility (Grades a+b) were significantly increased after treatment with Maca".  So Maca gives you more sperm, better sperm and more seminal fluid. And it definitely makes you feel more "full" down there below the belt and ready to ejaculate!
It's interesting that these men had no statistically significant changes in hormones such as follicle stimulating hormone, luetinizing hormone, testosterone, estrogen or prolactin. Somehow, Maca does its thing without significantly altering hormone levels. However, the researchers did speculate that perhaps Maca changed levels of free testosterone, which is most critical for us males, without changing total testosterone.
Maca has many other very desireable properties, including improved lipid profiles, lowering cortisol, aiding in muscle growth, boosting super-antioxidants and so on. I document these in my link on the Powerful Effects of Maca.
One other item of interest is whether or not Maca affects androgens. No study to date has found a statistically significant increase or decrease in testosterone or other hormones and Maca's actions appear to be in the aphrodisiac realm. One suspected compound is p-methoxybenzyl isothiocyanate, but this has not been thoroughly studied.
Maca has an excellent safety profile and we can take comfort in the fact that it has been eaten as a food for probably millennia by the Peruvians and Bolivians. It has been placed fresh on foods by the natives as well as cooked in various dishes. Peruvians consider it a food, although they recognize its "medicinal powers" as well. The only caution that I know regarding Maca is that one study on Maca showed a moderate increase in blood pressure at the relatively low dose of just 600 mg/day.  (Of course, work with your doctor whenever trying a new program or supplement.)
I take organic Maca from Navitas: Navitas Naturals Organic Raw Maca Powder and have had good results with it. I always cook it, usually through throwing it in my oatmeal.
Some people have trouble digesting the starch of the maca root, which can lead to stomach upset, gas and/or diahrrea. If this is an issue for you, you can order the gelatinized form, which is usually in the human studies by the way: Maca Power - 100% Certified Organic "Gelatinized" Maca. My understanding is that simple cooking does not break down these starches sufficiently for those who have trouble, whereas the gelatinized version handles that for you.
I should also forewarn you that Maca has a strong taste that is completely quite different from anything else you have probably eaten. I don't find the flavor objectionable but many people do. For many guys, though, the benefits far outweigh its strong aftertaste.
CAUTION: Maca is generally one of the most well-received supplements - although really it could be considered a food like fenugreek - on the forum and in emails. For example, this forum poster lists it as his favorite. However, I did get one forum poster who said that Maca gave him issues and actually lowered his libido and another that said it lowered his testosterone. You can read about that here. Maca reportedly can be stimulating for some and so some men cycle it, i.e. four or five days consuming Maca and a few days of rest afterwards.
Also, it is worth noting that some supplement manufacturers put Maca and Horny Goat Weed together in the same capsule taking advantage of their complementary effects on male physiology. Horny Goat Weed directly boosts Nitric Oxide and boost libido. Maca, through a complete different pathway, boosts semen volume and libido. It is a good combination that does work well for many guys, but you want to get a formulation that has an extract of Icariin (for the Nitric Oxide) and 500 mg of the Maca.
1) Urology,2000,55:598-602;J Ethnopharmacol,2001,75:225-9
2) Asian J Androl,Dec 2001,3:301-303
3) Food and Chemical Toxicology, Mar 2008, 46(3):1006-1013, "Maca (Lepidium meyenii) and yacon (Smallanthus sonchifolius) in combination with silymarin as food supplements: In vivo safety assessment"