I thought this movie was awful, personally. I had heard a lot of hype about it, so perhaps my expectations were too high. When my friend mentioned that it was much better viewed with the mindset that you're watching a grade-B '"so-bad-its-good" movie its more enjoyable, and I could imagine that. But as for the "psychedelic masterpiece" that I was told that it was, highly disappointing.
The philosophical implication of the film seemed to be that the pursuit of knowledge, understanding, and experience is fruitless and dangerous, and that one is much better off sticking to worldly matters. That did not sit well with me. What sat even less well was the film's treatment of the Amanita shamans who are depicted as savage, evil, black magickians. I found that to be highly offensive. The obvious association with John C. Lily's work also seemed offensive as, under that view, the "message" of the film and the depiction of the "doctor's" obsessive madness appeared as an invalidation of Dr. Lily's work.
It goes without saying that this movie left me feeling a good bit of unnecessary anger.
I have not read the book, but I was a huge fan of Network, a Paddy Chyevsky script. Supposedly Chyevsky also was very angered by the directions the film took and worked to get his name off the film, which he was unable to do.