I remember an “Oh wow” reaction on reading it when I was in my late twenties—already Wiccan, but still in that eager mode of scooping up new intellectual sensations (something I can still do when the stars are right).
This was in my pre-grad school stage. I did not even know that the author was female—after all, Robert Graves had written The White Goddess, and my only association with the name Merlin was the Arthurian one. (Apparently I was not going to the right conferences. I was not going to any conferences!)
There are lots of tributes to the book’s imaginal power (if not scholarship) at Jason’s post.