In Richard Pyle's article about deep stops he tells about fish collecting in deep waters of Hawaii. Most of the time, his catch was either dead or distressed upon surfacing, and he usually needed a nap after diving. One dive he had an issue which caused him to stop for a couple of minutes during his ascent. His catch was in perfect health after surfacing and he felt great. After more research he found this to be the case every dive.
Pyle's article is not talking about simply feeling tired--he is talking about the possibility of subclinical DCS. This is the origin of his deep stop system for decompression ascents (the Pyle Stop), not merely an ascent at a safe rate. He has since had second thoughts about it. He was a part of a presentation team at the DAN 2008 Technical Diving conference which cast strong doubt on the value of deep stops. I heard (but did not see first hand) that he has recently said that after a life time of diving, the only diving related concept named for him--the Pyle Stop--does not work.
I've always been amazed at how many divers say that nitrox makes you less fatigued, despite the fact that every course book tells them it's a placebo.
I have not read every text on Nitrox, but the ones I have read and the one I use when I teach do not say it is a placebo effect. They say that the fatigue benefit is unproven. There is a big difference. Saying it is a placebo is saying that it has been proven, and nothing has been proven.
I have seen several posts over the years in Scuba Board that refer to the only attempt I know to measure the effect, a test which showed no benefit. That test was absolutely absurd. I don't remember the details exactly, but it was something on the order of taking divers to something like 60 feet for 30 minutes for two dives and comparing the levels of fatigue. No one gets tired under those circumstances. Put people on a liveaboard doing 4-5 real dives a day for a week and check the difference, and you might have a real basis to compare.
As an instructor, I tell people that the effect is unproven through experimentation, but many people report that they do feel less fatigued when diving over time with nitrox. That is all we know, and all we can say for sure.