I've heard many personal accounts of single-dose MDMA transformations of an intensely powerful and lasting nature. One friend once compared it to the mystical idea of "walk-ins," souls that take over the bodies of people who have lost the will to live. This person said they walked into a club as one person, took MDMA, and walked out feeling as if a different soul now inhabited their body with an entirely different personality and emotional structure. The change was permanent. Luckily it was almost entirely positive. The "new person" was more sociable, compassionate, outgoing, and optimistic. The person who told me this story used MDMA only a few more times, and none of these subsequent uses produced anywhere near the same experience. It sounded like some kind of one-shot neurological rewiring.
The stuff I've heard about MDMA is far more dramatic and lasting than the transformative psychedelic stories about LSD, psilocybin, etc. that I've heard and read.
BTW I said the transformation is usually positive because while I haven't closely known anyone with a horror story I have seen a few from a distance. MDMA is not strongly physically addictive, but it can be psychologically so. The negative cases I've seen seem to involve the development of a psychological dependence causing heavy use. MDMA can be neurotoxic if taken repeatedly and heavily, and heavy long-term use can be very damaging.
It's a drug that I think could be beneficial if used with care but can be very dangerous to abuse. The radical transformation stories make me really wish it were studied more. We should really try to understand what the hell is going on there.