The summary tags this as "the most famous Science Fiction Novel ever written" and that's probably true, or close enough to true. Fittingly enough, it doesn't stretch for "greatest".
The initial premise is actually a good one: a human who has been raised by Martians come to Earth. It sounds like a good way to explore our society (rather, American society in the 1960s) using a fictional society to bring it into focus. And that's where the book starts out, and where it's the strongest, when the man from Mars is still getting used to this entirely different land he finds himself in.
The biggest problem the first half of the book has is exactly what kills the second half entirely: it's so preachy. In both halves, Heinlein wrote long speeches, lectures really, that one character (usually the older lawyer and probable Heinlein stand in Jubal Harshaw, later Valentine Michael Smith himself) will deliver to a particularly slow audience (however clever that character might seem in the rest of his or her interactions) who will only respond to a big chunk of speech with "Huh?" or "What?" This happens over and over again, through the whole course of the book, and frankly, it starts to drag. I just got sick of being lectured.
And maybe I wouldn't have minded being lectured quite so much if Heinlein weren't trying to sell me on a 60s free love fantasy. I have no doubt that this was much more progressive and revolutionary in the 60s. But as a modern reader, I can't help but notice how dependent this version of free love is on traditional gender roles. This reads less like the group marriage that Heinlein had intended and more like a shared harem. Women, despite being described as intelligent and capable, are essentially interchangeable here. They all talk and act in roughly the same manner. And then, late in the book, I find that one woman has changed her appearance to make herself look just like another woman. I guess to Heinlein, the fantasy here is that a group of beautiful, perfectly identical, perfectly interchangeable women would always be sexually available to the men in their lives. This is not my fantasy.
This probably goes without saying, but it's also entirely heteronormative, to the point of casual homophobia. Mike makes it abundantly clear that he's resolutely heterosexual, and so is his entire nest. I can deal with that. But earlier in the book, the worry that poor, innocent Mike would be seduced by (horrors!) another man are dismissed. Mike would sense a wrongness about an "in-betweener" and would never admit one that closely into his circle. Sigh.
That's a "product of its time" sort of issue. And while I'm considerably less enraged by finding that sort of thing in a book written in the 60s than in a book written today, I still intensely dislike and am disappointed by it. Heinlein could think ahead far enough to a world where travel between Mars and Earth could be relatively easy, but not far enough to a world where women could be something more than nurses, secretaries, or sexual partners. It's a shame, but he wasn't nearly as visionary as he thought he was being.