It's got a bit of a cheesy late fifties-early sixties vibe, some of the slang and the general misogyny.
But Heinlein deftly imagines an interplanetary and advanced future and how a human born and raised on Mars (by Martians) might be welcomed on earth. The concept of "grokking" (i.e., knowing fully, but also sexual consummating and cannibalism) was unleashed upon an unknowing America, and one must still only wonder at how many people have actually read the book as opposed to how many use the term "grok" fluently.
Too long and too polemical, the action really dries up in the (lengthy) middle of the book. Still, a fascinating novel of ideas.