DMT was a key tool in curing SWIM's depression. I don't know how DMT affects the mind physically, though, but others on here seem to have some understanding.
I tried to find the link to a great documentary I once watched about psychedelics and psychological therapy, but unfortunately I can't remember the programme title. Basically, there were experiments in the 60s with LSD in particular. They found it could really help people when they were directed through the experience. Unfortunately, the use of psychedelics was discredited by some other very brutal scientists who basically just dosed people up and locked them in a room alone for hours, which understandably had the opposite effect. Usefulness is completely dependent on providing optimal 'set and setting'. Incidentally, on that or another documentary it was explained how MDMA was developed as a drug for use in psychological sessions, such as marriage counselling, if my memory serves me correctly.
I'm told that studies by the Brazilian government concluded that ayawaska has a positive psychological effect on people in the 1980s, just before legalising its use (this is hearsay though as I've never myself seen the reports).