Interesting thread and one I feel I'm able to comment on. I did my Tec Deep Diving course this summer. During the course my instructor did a couple of things to see how I reacted to narcosis. On one dive he took me down the anchor line to 90 feet FAST. I kept right up behind him then we did our dive. When we surfaced he asked how I was. I told him fine. A guy who was also on the dive was going on and on about how messed up he was at the bottom. He came down the line at a slow pace. It was after the dive that my instructor told me he tried to get me narc'd.
A few more dives into the course had me at the 140 - 150 range. These dives were in the cooler waters of the St. Lawrence river. I felt absolutely no negative narcotic effects on those dives and thoroughly enjoyed the thrill of being at that depth (probably due to narcosis - the good kind).
Later in the season we were attempting a deep wreck dive. I was a bit nervous about this dive but not too worried. That changed once we hit the current at the beginning of the dive. We swam hard to the rim of a wall and hung on tight at the team assembled. When we went over the wall there was a down current that pushed us from 40 feet to 180 feet in just over a minute.
So NOW I'm Narc'd. That's Narc'd with a capital "N". Tunnel vision set in and my mask began to leak due to a problem exacerbated by the extreme pressure. I knew I had no business being in such a dark and cold place. Especially a place without air. There was actually nothing wrong with me, my gear (except for the leaky mask) or my supply of gasses. I just knew that I really wanted to leave. I looked at my instructor, made a circle with my left thumb and forefinger, stuck my right forefinger through the circle then pointed to myself. He took me by the arm and we made our way slowly back to the surface. As we ascended the feeling of "wrongness" at being underwater subsided. That evening was a reflective one. I can tell you I felt like crap at the reaction I had but in retrospect I needed it. The next day we did more Tec drills within the 130 foot range and the following day we did a 30 minute drift dive at 175 feet. I didn't feel narc'd on either of those dives.
I think a lot of what's been said is true and yet there's no way to really nail down narcosis. The narcotic effect of breathing compressed nitrogen happens on every dive yet, like alcohol, it affects people differently. Fitness, hydration, temperature, exercise, stress all affect narcosis but these all depend on the physiology of the diver. Sometimes narcosis is scary but sometimes it makes the dive exhilarating.
During my last dive of the season we were drifting for about 20 minutes at 155 feet. I was feeling a bit narcy and thought "ok, I'll unclip my SPG, shine my light on it, take a reading then clip it back. Keep my self busy in other words. Suddenly the channel narrowed and my teammate and I were shot forward like bullets from a gun. We looked at each other and each of our eyes said the same thing; YEEEEEHAAAWWW!!! That's when having a bit of a narc-on can really make the dive fun.
Still, this year's goal for me is Trimix!