Ah, Stranger in a Strange Land. Number 3 or 4 on my top five favorite books list. Get the unabridged version if you can find it. It contains a few scenes the make a great book excellent.
I once read some, if not all, of Foucault's Pendulum. I found it quite frustrating to read, Although it's been quite a long time, I seem to remember not being quite able to follow the logic of his narrative. It seems to me that he wandered off on tangents that made the story rather diffuse. (Which kind of reminds me of the Rings trilogy, although I'm not sure that's an apt comparison.)
I agree, the unabridged version of Stranger in a Strange Land. When I read the edited version I found it a bit confusing-- Important things had been cut out.
Have you ever read Gateway? Excellent book! Don't read ahead, the last line will blow your mind.