The book helped me and my friends more than we can probably imagine.
This was when we were maturing teenagers and took LSD with fairly gay abandon (1999-2001). Our group was generally fairly smart, well read, and found the idea of the psychedelic experience fascinating, so that mindset helped as we moved into a period when we would have some extremely powerful acid trips.
I remember looking for The Psychedelic Experience from the local library after reading about it, shortly after my first trip - which hadn't been spiritual, but had hinted at the possibility. The book showed up on the local library intranet as part of the local collection, but it was not kept in the main library, it sat in the archive in the cellar, so I had to specifically request it in person.
Turned out the relatively young librarian I interacted with, knew all about Leary et al's book, and reacted enthusiastically when I asked about it. He said he and his friends had been really interested in it when he was my age, and wasted no time in ensuring I had it in my hands. I remember when he gave it to me - it was in A4 format with a polythene cover - and how the whole experience made me feel as if I'd uncovered something mysterious, taboo, rarely spoken of, and only in hushed tones at that.
It structured the trips we had after that in a way that I'm almost definite made them more manageable and navigable, otherwise we would have been just a trio of tripping fools with the odd revelation. It probably kept us safe, too.
Looking back, I know now the first bardo clear light, second bardo, and third bardo 'confrontation with reality after rebirth' are entirely human constructs, but this doesn't take away from their utility in the context. In my mind, Leary, Metzner, and Alpert did the psychedelic community a massive service by publishing this book, despite its obvious historical contingency. I'm sure it prevented many confusing and earth-shattering tight rope walk trips from going completely south.