I bought this as an audiobook and I'm about 1/3 of the way into it.
It is a little dated, although not as bad as I thought it would be. There could still be an advanced civilization on Mars, we don't really know yet. What dates the book more is what is missing from it -- email, internet, wireless, nanotech.
That aside, the writing is wonderful. Heinlein's dialogue is some of the best I have ever read and the characters are fascinating. The plot immediately pulled me in. Heinlein has created a convincing alien civilization and reveals it slowly.
I can't comment further, yet...
Re:Stranger in a Strange Land
For your next post: You shalln't comment; you shall grok.
Good Book. Says more about ourselves than science fiction stuff; all good science fiction books SHOULD, even if Humans aren't present. Even in his juvenile/adventure fiction, Heinlein knows how to comment about humanity. I should pull it out. Still like The Moon is a Harsh Mistress best of all his works, but I fully grok Stranger in a Strange Land.
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Life is a lot like caving: Most of the time you grope around in the dark.