It might be of some interest to know that the decompression chamber demonstration was in use in the RAF as early as 1942, when I went through my Spitfire OTU at Hawarden. I think it was a mobile unit (similar to the mobile x-ray units which toured the country to detect early tuberculosis).
I think they took us rather rapidly to 20,000 ft (and a Spit I could certainly climb to that in 10 mins) before the "guinea pig" took off his mask. My experience was exactly as described by Avtrician (#7) and john_tullamarine (#8).
It's a long time ago, but what I found most impressive was the insidious onset of the symptons; you have no distress or fear of what you know must be happening to you, you simply slip off into oblivion without a care in the world.
Your "buddy" beside you, on oxygen, watches you like a hawk, and the moment you slump off motionless like a hibernating bear, clamps your mask back on with 100% oxygen.
You "come back" in reverse order to the way you went off. Many people at first refused to accept that they had been "away" at all, even when confronted with the meaningless jumble into which their handwriting had deteriorated.
On a macabre note: I have often thought that this might be an ideal way to commit suicide (if the Law allowed !) or to carry out a death sentence. Cheap, easily available, no pain or fear involved, as easy as falling asleep when dog-tired.
Danny42C.