Sorry Pace, I only just saw the link you posted which was very interesting reading.
The author did state he flew up 18,000 ft maybe 8 times per day, and that it was tiring and he knew his test figures were not accurate. Sounds like he did indeed have a pretty good tolerance, and it also appears that he was a little aclimatised from his regular climbs. Even his climb to 20,000ft sounds pretty dodgy - I doubt he stayed up there for very long. I particularly enjoyed his reference to drag on a cigar at 20,000ft.
I've been fortunate to have experienced hypoxia many times, fortunate only that it was in a controlled environment, with a Doctor present. The effects for me are confusion, difficulty with basic maths, poor memory, selective hearing (my ex-wife could have already couched for those points). I also noticed a slight tingling in my fingertips, and most scarily, a euphoric feeling. And then the shakes. In fact I felt great - why are those other guys puting their masks on, when I've never felt so great? I likened it to having my drinks spiked with very strong alcohol without my knowledge. The worst point is though, that sometimes I can catch it, and decide when I need to put on my mask, and hopefully actually achieve it. Other times though - no good, I simply would have collapsed.
Another scary aspect is when the 100% oxygen is placed back on, the hypoxic effects actually worsen initially, with some users actually trying to again remove their masks.
Only once to my knowledge have I experienced this in an aircraft. just the initial symptoms, including tingling in fingertips and lips. I voiced my concerns and the flying pilot decended. Once on the ground we discussed it, and turned out that he knew nothing about hypoxia. He probably concluded that I was just a bit of a girl...