Probably the prime benefit of Citrulline supplementation is showing itself to be increasing erectile strength and hardness factor. In fact, in my forum some of the recent comments from men with erectile dysfunction have been strongly positive. One man with apparently rather severe erectile dysfunction commented that he had an erection of sufficient quality for penetration, something that clearly had been missing for him for a long time.  In fact, he mentioned that he had tried other supplement combinations without any success, so L-Citrulline appeared to have the most horsepower for him.
L-Citrulline is somewhat of a newcomer to the erectile supplement market. Researchers have for years concentrated on L-Arginine to raise nitric oxide and results had stalled. Arginine had had lackluster results and in one study even appeared to be dangerous for some men with advanced heart disease - see my link on The Pros and Cons of Arginine for more information.
However, just when "all hope seemed lost", L-Citrulline showed up as the real superstar in recent latest. What is interesting is that L-Citrulline is a precursor of L-Arginine. So one would not expect it to really do any better or worse than Arginine supplementation, right? Actually, the opposite turned out to be case. Taking L-Arginine directly has issues, which we will discuss below, that make it less than ideal. Arginine had to be taken in overly high dosages (5 grams +) to have an effect. For example, one study found that about a third of men, probably those with endothelial dysfunction as we will discuss below, had significantly improved erections with 5 grams of L-Arginine daily. 
Unfortunately, 5 grams approaches the daily amount given (9 grams/day) in the above-mentioned study of Arginine where six men died. Of course, Arginine can be used successfully when at much lower doses (1.5 grams/day) when combined with other supplements such as Pycnogenol. But all of this goes to show that L-Arginine has definite issues when used by itself.
One of the big issues with ingesting L-Arginine directly is that it is metabolized in the liver by arginase. And in men with heart disease, arginase is upregulated or more active. This means that more Arginine is converted to the amino acid ornithine, which, as mentioned previously, can potentially lead to herpes outbreaks and other issues.
L-Citrulline, on the other hand, seems to produce equal or better results at much lower dosages. For example, one 2011 study found that a mere 1.5 grams of Citrulline increased the hardness factor of a significant number of men.  One of the reasons that so much less is required is that Citrulline supplements completely bypass the liver and the whole arginase conversion process. One study put Arginine head-to-head with Citrulline and showed that Citrulline was the clear winner. 
One beautiful thing is that you can easily get this dose through food, namely in watermelon. One study found that an average dosage of watermelon, although the content can be quite variable, was 1.9 mg of Citrulline per gram of weight.  This may not sound like a lot but you have to realize that a cup of watermelon is about 154 grams or 290 mg of Citrulline. I don t know about you, but I can easily down a few cups, and voila! you ve just consumed a clinical dose, making watermelon a true super erection food.
Yet another study  found that drinking enough watermelon juice to get 1 or 2 grams of L-Citrulline daily led to 12% and 22% increases in plasma Arginine levels, respectively. This will translate to increased nitric oxide levels, especially for most middle-aged and beyond men. CAUTION: It also increased Ornithine levels by 18% as well.
What I have used is Citrulline in powder form from Source Naturals: Source Naturals L-Citrulline Powder, 100g. This works very well for me and seems to be the most powerful erectile dysfunction supplement for me. The powder, at least in my opinion, actually tastes half way decent and is probably the cheapest option. I have taken between 1-3 grams/day in 3 divided dosages and all of them seem to help. Talk to your doc first of course.
And it is no wonder that Citrulline works so well for me : it is now thought of as an excellent solution for men with endothelial dysfunction. The reason? It actually acts as an alternative source for arginine - rather than the typical endothelial pathway - and thus provide an excellent way to increase your baseline nitric oxide levels even if you arterial issues. In addition, you lose some of your nitric oxide just from aging, so this is a way to compensate.
Remember: even Viagra and Cialis really do not "create" nitric oxide but rather work on the enzyme that breaks it down. Viagra and Cialis simply allow nitric oxide to stay in your system longer, because it is broken down less quickly.
However, this is one of the reasons that Viagra, Cialis and Levitra do not work for many men with endothelial dysfunction. In these men, the lining of their arteries and blood vessels is so damaged that they do not put out enough nitric oxide for these PDE5 Inhibitors to act upon. L-Citrulline can substantially help such men by actually providing more substrate for these medications to act upon.
CAUTION: Always check with your physician before trying any new supplement with another medication. Example: One forum poster said that Citrulline gave him a migraine (since any vasodilator can do that). 
Or, better yet in my opinion, L-Citrulline can act as a source of nitric oxide for other more natural NOS (the "nitric oxide enzyme") solutions such as Ginseng, Pycnogenol, exercise and food. This, by the way, is one of strategies behind the nitric oxide boosting supplement that has recently been all over the web called Neo-40. It is an interesting combination of L-Citrulline and Hawthorn Berry Extract that does just this. One of the foremost nitric oxide researchers in the world, Nathan Bryan, helped design it and so we know we are onto something here. A couple of the guys on the Peak Testosterone Forum have taken it with decent results.
Citrulline also has excellent potential for improving general arterial health, which always benefits erectile function as well. How does it do it? By slowing down the progression of arteriosclerosis. That s right, an animal study showed that by increasing nitric oxide and blood flow and reducing superoxide (a potent free radical) gene expression, it could actually greatly help in the batlle against hardening of the arteries. 
The short term benefits are hard to argue with. As I mentioned, one poster on a forum thread said that L-Citrulline restored his morning erections. And yet another poster stated "I have only been on the Citrulline for 2 days and it appears to be marvellous stuff- practically the equivalent of having an explosion in my pants!"
DOSAGE: The highest dosage that I have seen in a study is 3 grams per day. However, I would recommend trying a lower dosage, 1-2 grams to start out with . Keep in mind that even a few cups of watermelon will likely help. There are also many excellent sources of L-Arginine in food, which tend to have a nice balance of both Arginine and Lysine (important for herpes control). For example, an egg white has almost equal levels of Arginine and Lysine (200 mg). Rice protein, on the other hand, has about a 3 to 1 ratio of Arginine to Lysine.
2) Urology, Jan 2011, 77(1):119-22, "Oral L-citrulline supplementation improves erection hardness in men with mild erectile dysfunction"
3) BJU Int, 1999 Feb, 83(3):269-73, "Effect of oral administration of high-dose nitric oxide donor L-arginine in men with organic erectile dysfunction: results of a double blind, randomized, placebo-controlled study"
4) Journal of Chromatography A, Jun 2005, 1078(1-2):196-200, "Determination of citrulline in rind"
5) Nutrition, Mar 2007, 23(3):261 266, "Watermelon consumption increases plasma arginine concentrations in adults"
6) PNAS, Sep 20 2005, 102(38):13681-13686, "L-citrulline and L-arginine supplementation retards the progression of high-cholesterol-diet-induced atherosclerosis in rabbits"
7) Br J Clin Pharma, 2008, 65:51-59, "Pharmacokinetic and pharmacodynamic properties of oral L-citrulline and L-arginine: impact on nitric oxide metabolism"