Nitric Oxide has proven benefits to anyone who lifts very heavy on a regular basis - e.g. someone doing High Intensity Training five days a week for an hour or more each day. The benefit is three fold:
1. Increasing blood circulation, thus pushing more blood into the muscle for a better pump. There's more to a good pump than the feel, as the increased blood pushed into a muscle is part of the process of triggering adaptation and anabolism within that tissue. Pushing blood also aids in faster ATP recovery. This means you push heavier weight for longer periods of time, and you only grow bigger if you grow stronger. Can't get one without the other.
2. Nitric oxide triggers the release of adrenaline, giving you, once again, greater strength during heavy lifting.
3. Increased oxygen within muscle tissue aids in recovery, and can let you workout with greater intensity more often and for longer periods of time.
The studies that have been conducted, and published, have monitored subjects involved in both resistance training, and endurance training, and the benefits to both groups are evident in all clinical studies conducted since as early as 1996. Measurable, and frequently impressive, increases in dense, lean muscle tissue have been experienced even in control goups dosing at very low levels. And, unlike steroids, the gains are maintained as long as the subject continued their same level of training intensity.
Loading it is tricky, as you have to determine your tolerance based on slowly increasing intake over a period of five days. NO is created through the conversion of L-Arginine which you can consume too much of, resulting in fatigue, gastritis and nausea. It is most beneficial if taken twenty to thirty minutes prior to intense training, and on an empty stomach.
Not everyone can tolerate it, and most of the commercial products on the market contain added stimulants - caffeine - in order to increase the jacked feeling subjects get, and the "focus". There are a few that do not contain stimulants.
It also appears to help in pushing pre-workout creatine into the muscle, as well, so that's a bonus, and the reason many NO products contain some creatine.