I had one reader write in with a supplement cocktail that he had created to help get him over "middle-aged erectile malaise." He was having mild erection problems but was very disturbed at the problems. To help get him over the problems he was having, he read through my site and then wrote me the following:
"I started the diet Tuesday and man I'm starting to feel a difference already. I also made a trip to the grocery last night and the Vitamin Shoppe . I took one of each of the Horny Goat Weed, Maca, L- Arginine, Pycnogenol and Ginseng and man let me tell ya, there was a difference this morning when I woke up! I felt 16 again ! Lets just say that the wife now knows that it wasn't all in my head. 14 years together can sometimes make ya forget details. Ok enough of that but WOW!"
It's a little hard to miss the enthusiasm there, eh? Between nitric oxide promoting foods and these supplements, it actually brought some enthusiasm back to the bedroom apparently. Of course, one of the most notable things about his formula is that he essentially threw every supplement together all at once and that begs the question, "Is that a good idea? Or a waste of money?" In fact, he wrote and asked, "Also, taking all the supplements together, is that what you recommend? It works."
NOTE: Always check with your doc first. Some supplements, such as Horny Goat Weed, can interact with certain medications for example.
First of all, researchers have looked at "supplement cocktails" - something I'll get to below - with considerable success. However, it should be pointed out that in the case of Ginseng, Horny Goat Weed and Maca, solid results were achieved with just one supplement. So one does not necessarily need to combine supplements to achieve good results.
That said, though, combining supplements does actually make sense in some cases because they work on different erectile and sex-friendly pathways:
1. Maca. This Peruvian superfood works on your libido (and semen volume). Scientists are not sure exactly how it works.
2. Horny Goat Weed. HGW (epimedium) has a PDE5 inhibitor in it (Icariin) that works on the enzyme that dissipates Nitric Oxide in a manner similar to Cialis and Viagra.
3. Korean (Panax) Ginseng. Production of Nitric Oxide by the endothelium is only step one in the erectile process. As the Nitric Oxide spreads into the blood and smooth muscle cells. Once inside the muscle cells, it triggers the release of an enzyme called guanylyl cyclase that in turn increases cGMP levels and it is cGMP that induces the relaxation response in your arterties, allowing them to expand and blood to enter the penis for an erection. What does this have to do with Ginseng? Several studies have shown that Ginseng increases cGMP. 
4. Pycnogenol. Pycnogenol works on yet another pathway. Nitric Oxide is released in the endothelium by shear forces and from various receptor sites, acetylcholine probably being the most prominent. Pycnogenol boosts significantly the Nitric Oxide released due to its action on this acetylcholine pathway.  This same study found a 42% increased relaxation of the arteries from Pycnogenol as compared to controls.  (Read this link on Pycnogenol and Erectile Dysfunction for more details.)
5. L-Arginine. L-Arginine is the precursor, i.e. the building block for Nitric Oxide and so it would stand to reason that consuming L-Arginine directly would increase Nitric Oxide levels. ADMA competes with L-Arginine and so consuming L-Arginine also increases the all-important L-Arginine to ADMA ratio for middle aged and beyond males. CAUTION: Please read my L-Arginine Discussion for potential side effects with L-Arginine.
So taking a supplement cocktail makes some sense in that these enhance your artery-relaxing, erection-boosting at every step along the way. Of course, for many guys this may be overkill because improving just one or two pathways may be sufficient.
In fact, the most important point in the regard is that many foods boost Nitric Oxide as does exercise. Thus, taking supplements for many guys is simply a cheap substitute for eating right and exercising. However, some guys need some short term help while they're getting back in shape and transitioning their diet and a cocktail such as the above may be worth considering.
NOTE: For safety considerations, please see my links on these individual supplement and talk to your doctor.
1) Biol Pharm Bull, 1996 Nov, 19(11):1434-9, "Effects of dietary supplementation of lipophilic fraction from Panax ginseng on cGMP and cAMP in rat platelets and on blood coagulation"
2) Hypertens Res, 2007 Sep, 30(9):775-80, "Pycnogenol, French maritime pine bark extract, augments endothelium-dependent vasodilation in humans"