A destist I went to in 2012 to fix a broken tooth had it available. This was the first and only time I have used it. It was wonderful.
Everything horrible that was happening in my mouth — all the drilling, injections and so forth — seemed to be happening many feet away. It all seemed like a dream.
The piped-in music seemed to be an unending loop of some musical phrase from an 80s song I couldn't quite remember. I had the pleasant sensation of falling through an infinite void, which I hadn't experienced since I was a child.
And when it was over, the gas was turned off and I left with a great feeling of calm.
In short, you should ask for it.