I recently received an interesting email from a middle-aged reader. Basically, he felt strongly that even small amounts of Arginine, or more correctly L-Arginine, made dramatic improvements in his erectile strength. A preliminary look at the studies might lead one to believe that this was a placebo effect, which can be significant with erectile and hormonal issues. This is especially true since the amount that he was taking was relatively small as well - less than 3 grams daily for sure.
L-Arginine has simply not proven so far to be a heavy hitter in the nitric oxide/erectile dysfunction world: studies have been lackluster at best. The one exception to this is the study where a little Arginine was coupled with Pycnogenol and provided an excellent and relatively safe supplement solution for guys with erectile dysfunction.
However, in spite of Arginine's checkered past, the research definitely leaves room for it to be a significant erection-booster for some guys. For example, one study  of patients with Type II diabetes found that just 3 grams daily of L-Arginine dropped blood pressure very significantly, from an average of 135/87 to 123/81. This is impressive and resulted from a substantial increase in the "erection chemical" nitric oxide. (L-Arginine is the precursor of nitric oxide.)
The study authors were not looking at erectile issues, but the extra nitric oxide would have undoubtedly helped the male participants noticeably. So a key point here is that for those with type II diabetes and/or hypertension, Arginine may produce much more powerful results than for healthy individuals. Of course, a huge percentage of guys in a Western society that are middle aged and beyond are diabetic, prediabetic and/or have some hypertension or Metabolic Syndrome. So L-Arginine may give a nice increase in erectile strength for such individuals.
CAUTION: Talk with your doctor first and read my link about Arginine and some of the reservations and cautions that one should consider first.
The reason that L-Arginine helps diabetic men so much? Animal studies have shown that L-Arginine raises nitric oxide levels much more in diabetic animals.  In other words, "a little goes a long way" for those losing their blood sugar control.
There is another mechanism by which Arginine may help the typical middle and senior-aged guy: ADMA competition. I hope to write a page on this soon as it is a fairly involved subject, but the important thing to realize is that ADMA occurs naturally in our blood and directly competes with NOS, nitric oxide synthase (the enzyme responsible for nitric oxide production). Thus ADMA can be a significant drag on male endothelial function as the decades roll by. In fact, ADMA is linked to heart disease, arterial plaque, endothelial dysfunction and possibly even erectile dysfunction as well.
Research has shown that Arginine actually helps with ADMA-related issues very significantly. Again, this may be yet another reason that some guys get solid results from L-Arginine.
1) Journal of the American College of Nutrition, 2002, 21(5):422-427, ""Oral Arginine Reduces Systemic Blood Pressure in Type 2 Diabetes: Its Potential Role in Nitric Oxide Generation"
2) DIABETES, NOV 2005, 54:3282-3287, "Reduced Nitric Oxide Concentration in the Renal Cortex of Streptozotocin-Induced Diabetic Rats"