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...