Twice, and they were completely different experiences.
First was gas at the dentists for taking 3 teeth out as my mouth was overcrowded. I was kind of asleep, I could hear people's voices in a really trippy flanged way, and I could vaguely feel some tugging at my jaw (but no pain). The gas tasted awful.
The second was for an operation at hospital after an accident (requiring 6.5 hours of microsurgery). It was like jumping forwards 7 hours in time, literally counting the seconds after the anaesthetic went in at night, then immediately waking up in broad daylight. It is completely unlike deep sleep (where you still are aware that time has passed).