Haha, a course I did at uni a year ago had one lecture/two hour discussion concerning the same topic (the lecture actually started with the professor admitting to his experiences with LSD and magic mushrooms :lol
. Don't have any texts on the subject though, nor any notes, and it's probably too late for that now anyway.
However, from what I remember by heart, and what I believe myself, is that psychedelic experiences are valid experiences because they can cause significant alterations in a person's personality, way of life, approach to life, mental state... pretty much everything. How can something that can have such a great impact
not be a valid experience, for the person experiencing it?
Second (I think this is along the same lines as what IJC pointed out), what makes the "ordinary" configuration of the brain more valid than an "altered" configuration of the brain? Had evolution progressed differently, reality might have looked extremely different. And if it would be possible to compare this reality that would be the product of this different progression of evolution (let's call it reality X) to our contemporary "ordinary" reality, then reality X would probably resemble much more closely a "psychedelic reality", in the sense that it would be extremely different from our current ordinary reality, than it would resemble our current ordinary reality. This only further brings doubt to the notion that
any experience of reality is more valid than any other experience of reality.