My personal experience with narcosis (I dive deep on air regularly up to 60 m) :
- my very 1st time : 38m on my PADI Advanced, in the Med, during summer 1991. I felt good ! Sooooo good ! The site was lovely, water was clear and warm. Didn't behave erratically and very quickly, it rang a bell to me : don't you remember Cousteau's films you jackass ! You're narked ! So watch out. While debriefing with my instructor, he told me my behaviour had been ok, but he has his doubts cause I was obviously enjoying myself a bit too vividly.
- usually below 35m to 45m everything is ok, but I've noticed my brain is bit slowed down. Slowed down, but not impaired. Already had to assist out of breath buddies at those depths and we've managed it allright to the surface,
- below 45 m and up to 60 m : there it's more like slow motion movie, I usually try to keep on edge, cause I know I'm narked even if I don't feel it. When water is clear and warm, it's ok.
But when it's cold and dark, things aren't so easy. Remember a dive in the Biscaye Gulf in July : while changing on the dock (above 30°C), the sun roasted us, then 45' of uncomfortable navigation cause of the rough waves, then 18' at 50 m in a 12°C water, dark as pitch. Usually, it's a dive I enjoy very much. My buddy was a friend, and a very able diver, everything should have been terrific, but I've absolutely no memories of that dive ! I know I did everything as planned (we dove run time : air + 50% EAN) and my buddy told me I was answering his hand signs correctly and that we communicated easily all the time, but don't ask me what I saw, I haven't the slightest idea...
Another unpleasant experienced with narcosis : July, blue water and perfect conditions in the med. Dive planned on a rock which rises from the bottom around 50 m up to 10 m more or less. My instant buddy was a 4* CMAS diver (or DM if you prefer to call him so), myself I'm a 3* CMAS diver. We dove straight to 50 m, not particularly fast, easy going I'd say. Then, all of sudden, my 1st went amok, I heard a blow out, then bubbles coming out very fast from it. Couldn't reach my valve, asked my buddy for help, I didn't feel easy but didn't panick and gave him the right hand signals correctly : he just kept looking at me with a blank stare... So, I called off the dive and signaled him 'going topside'. Thanks god he followed ! But while waiting for the boat to come (in less than a minute) I got a very harsh bunch of rebukes for spoiling his dive for no good reason. He denied I ever had a problem with my reg... Cannot say we were on the best of terms after that...
Hope it'll help.