My experience was that I soon began to feel tingling in my hands, as if they were falling asleep. A short moment later I began to get dizzy, and my vision went grey around the edges. Each of the three times I have attempted this method, that's as far as I got before I flinched and removed the bag.
I did not experience any pain whatsoever, and no real discomfort aside from the mild tingling in my hands.
I do not believe I have suffered any ill effects from three aborted attempts. I had a bit of a cough for a couple days after my third attempt, but I can't say for sure whether that was attributable to the N2. It concerns me only in that this method demands that you be able to breathe freely in order for it to be effective, and I don't want to have ruined my chances to use it later, when I have summoned my courage.
My greatest difficulty is that I have been very stressed during my attempts, and could not keep my breathing steady and deep. This undoubtedly lengthened the amount of time it should have taken for me to black out.
In the stress of the situation, I experienced time dilation —in other words, I can't tell you how long it took for me to feel dizzy; it might have been five seconds, it might have been a minute and a half. It wasn't very long —but it was long enough for me to flinch.
From my experience, this is as painless, comfortable, and potentially peaceful a way to ctb as it is rumored to be. The only drawbacks that I can see are that it requires a lot of equipment —cylinder, regulator, hose, bag— and that there is a surprising amount of time for your survival instinct to kick in, or for second thoughts.
As I wrote at the beginning, I hope other forum members can fill in some of the blanks I have left —specifically, I have seen a lot of questions about tank sizes outside the US that I cannot answer.
I don't wish anyone good luck at catching the bus. I wish we could all find our way to a joyous and satisfying life that would make death an unfortunate reality rather than a longed-for relief. But all of us here know that the Fates aren't always that kind, and so I wish you all the most peaceful relief from your pain that you can find, and if that peace is brought by Death, I hope Death comes gently.