Goodreads ate my first review.
Trying again.

If you don't have mescaline handy, then a good way to experience this book is to have an old guy read it to you. The BBC produced an audio version for that purpose.

Huxley gives a literate description of his experience with mescaline, then provides convincing (to me) arguments as to why its use should be permitted. Fuddyduds will not be convinced.

The book provides some insight into why we need doors to perceptions other than the survival driven, symbolic, perceptions we are used to. The doors available to us are all kind of boring: church, alcohol, tobacco. He mentions a couple others which may have been legal to him.
For Huxley, mescaline was a superior door to all of those.
He felt the main problem with general usage is that its effects last for eight hours. Not a casual experience.

I wondered if Carl Jung had done anything similar. Answer... Yes. He even illustrated it.
http://www.nytimes.com/imagepages/2009/09/20/magazine/20jung.3.ready.html