Other reviews address the content, significance, imagination etc.
No one else has mentioned this yet, but I am most impressed with just the in-depth quality of the writing craft itself, once you get past some strange quirks, like the sudden jumps among different first- and third-person perspectives.
Parts of it might read as if they are just "stream of consciousness", but they were certainly not all just streamed out that way onto the page, without significant work ... when you step back, and look at the assembling/disassembling/re-assembling of so many intricate parts, into a twisted non-Euclidean curved pyramidal whole, the craft and work involved is quite impressive.
Also having "been there" a few times, many years ago, I can say that their vivid verbal portrayals, of some extremely emotional psychedelic hallucinogenic schizoid states, are right on.
(ignoring the actual content of the specific trip, I mean, just focusing on the verbal portrayal of the emotional state, it reads like something out of my own long-ago experiences, except that they are much better writers than I could hope to be, they actually managed to get it written down in words, in a reasonably effective way!)
Plus, so many of the characters etc. are familiar from my youth, it is a huge nostalgia trip for me too, a nice ride through familiar old stomping grounds of my youth. That is, back around then, circa 1972, I was a 12-year old pre-pubescent H.P.Lovecraft freak, Abbie Hoffman freak, science fiction freak, etc., had a crazy mother totally into Atlantis and all that fernando POO, and at the same time also had friends in the neighborhood, who were into Aleister Crowley, Anton LaVey Satanism, etc. Consequently I knew a lot more about the Satanic Bible, for instance, than I ever did about any of the Christian/Jewish mis-translations.
So, it's like, the story of my tribe, it's like I could have been a very minor character, in the book. And there are a lot of us wandering around, still dazed & confused out here somewhere at least in the back of our minds .... so I see, this book is for US.
This is a book I SHOULD have read, back then when it was new. But that's OK, I lived it instead... and the book is still a great read, even today, over 30 years later.