I've only done it once and then volunteered to take a second "hard mode" run at the end of the training with some of my idiot friends, where we left our gas masks outside.
It simulates suffocation although you certainly won't die to temporary exposure like this, and every so often some kid walks in and discovers he's naturally immune to its effects! I definitely was not one of those kids.
You feel it burning your skin under your uniform the moment you walk in, and then as you take your first breath (which you would've held had they not forced you to speak at length) all your senses just go nuts as you compound the problem by trying to get air. The burning spreads to your insides, which sucked.
You can breathe immediately once you're out and after 10 minutes you're pretty much fine. The annoying thing is that the gas crystallizes on the inside of your gas mask so when you don't clean it and then put it on later you gas yourself again :D
It'll fuck up kids who leave their contacts in though. All in all 6/10, would not recommend save to see your friends' reactions.