I have, sort of. I've worked HazMat most of my life. One of the jobs I had years ago involved neutralizing a large pit of acid. It was just a huge pit in the ground with a roof over it. From the outside, it just looked like someone had pulled the roof off of a house and set on the ground. There were only two openings, one at either end, so it was completely enclosed. The method here was to send the two youngest (and therefore invincible) guys into the pit with acid suits and full faced respirators, with buckets of soda ash, we walk around in it and stirred it up while we sprinkled the ash around. Safety standards back then were not what they are today. Anyway, the people in charge realized that there would be a reaction with gases betting released, hence the respirators, but no one considered the possibility that the gases might be heavier than oxygen. Which they were. We didn't know what kind of acid it was but this was an old fertilizer plant, so probably nitric. Which means the gas was most likely nitrogen. Whatever the case, we got into trouble when we realized that we were both getting rather lightheaded. We tried to leave, but the only way out was up a ladder and by the time we got to it the other guy, we'll call him Rick, could only get about half way up before he just couldn't move anymore, which left me leaning on the ladder at the bottom, completely unable to help, as I was in the same state. Luckily, our foreman was a lunatic and he jumped in and pulled us out. You are absolutely not supposed to do that because you are just as likely to end up in the same trouble as the guys you're trying to save.

The experience with the gas was not unpleasant. I should have been terrified, but was mostly just mildly concerned. The only real effects I remember feeling are the lightheadedness and being really sleepy.