I used to use it for the really difficult things, like the dentist. I tried using it when I was in labor/delivery with my daughter and that was the beginning of the end of it for me. I remember laying back and trying to cope with a contraction and then opening my eyes and three hours had passed, when it only felt like a minute or two. I can't recall most of the day, only segmented pieces. I wasn't on any drugs for pain when I was losing time. The whole day had this surreal feeling, like I had dropped acid (LSD) or something. I think it probably had more to do with all the crazy hormones and chemicals that are naturally produced to cope with labor. The disassociating stopped working when the pain was unbearable. It was very jarring to go from "not being there" to be in the worst pain I have ever experienced and absolutely terrified. They gave me narcotics soon after I reached that point.
When it did work, I could choose not to emotionally be somewhere and I wasn't. I didn't even realize that I could do it until I wasn't able to do it anymore. I really wish I could still do it. It was so very, very helpful.