Well..NO is merely just a vasodilator...something you will NEVER feel effects from. All it does is makes ur circulatory system (veins and arteries) vasodilate, aka allow more blood flow. The reason it's so popular is b/c it gives you an unnoticeable pump, due to the fact that when you're working out, you're obviously using your muscles which sucks ur blood flow to help keep the muscles in use well oxygenated so that u don't get anaerobic build up, aka lactic acid build up (that horrible burning sensation). So I wouldn't really call it a placebo affect, but it's not creatine, sugar, or caffeine or anything like that that will give you an energy boost.
As far as products. pure NO pills and what not is merely just that vasodilator effect...I quit that crap b/c like you said, it's unnoticeable. Instead I now use NO-Xplode which is a combination of NO, creatine, caffeine, etc. and that stuff WILL give u a boost of energy and keep the pump going. I wouldn't call it a placebo either, b/c days I don't use it, I get into a mindset that's less motivated, so i'd say the caffeine, creatine etc mixed with the NO gives me that rush to hit it hard in the gym.
Conclusion - pure NO is NOT going to change ur mindset or show any noticeable change in ur workout b/c it's completely internal.
Conlusion #2 - NO mixes like NO-Xplode WILL give u a noticeable change during ur workout. More mindset than anything else.