This review will contain spoilers!
In many sentences:
The first half or so of Stranger is quite good. The idea of a human so foreign that he must learn all the aspects of language, culture, and history that we take for granted is very clever. Furthermore, Heinlein executes on this clever idea masterfully. The ideas the book presented in the first two major sections were fascinating and enjoyable to read. Throughout this part Mike was truly a stranger to everyone he met. Jubal, meanwhile, was a sharp-tongued, funny, intelligent character whose open-mindedness was second only to his skill with an argument.
Here's an exchange between Duke and Jubal that I think captures the personality, wit, and wisdom of Jubal (Jubal starts by asking if Duke got anything out of thinking and Duke responds): "Yes," said Duke, "I've decided what Mike eats is his business." "Congratulations! A desire not to butt into other people's business is eighty percent of all human wisdom." "You butt into other people's business." "Who said I was wise?" (171).
Of course the "but" weighing down this critique is the fact that the rest of the book is heavy-handed and dull in comparison. What makes the first half so much better than the second? Mainly, while the first half is telling a cohesive story, with a well-paced plot, the second half is almost completely devoid of any organized story. Once Mike is free from his imprisonment in the hospital, and safe from political or economic challenges his inheritance created, the plot completely falls apart.
Instead of continuing a story, Heinlein decides to use the Man from Mars and the remainder of the book as a soap box for his ideas on open-minded sexual liberation. Regardless of my thoughts on the topic (which are that it sounds like a pubescent fantasy), my main issue is the means by which Heinlein gets the point across. For most of this section of the book, it's nothing more than one character after another justifying why the sexual liberation that Mike is helping them achieve. It is heavy-handed, annoying, and about as subtle as Pullman in The Amber Spyglass.
As a reader, I am always hate it when an author uses characters to come right out and tell me everything I'm supposed to learn from reading the book. Why couldn't the same point have been portrayed through an actual story, rather than a character stating it explicitly? The ultimate result was that I had to force myself through the last half of a book that I started out loving. In fact, I am probably even more disappointed with the end since it started off so strong. Another thing that makes it even worse is that Jubal is completely stripped of his integrity as a character by Heinlein's use of him as nothing more than a mouthpiece for various ideas on sexuality.
I do wonder if Heinlein was aware of this criticism, and attempts to justify it from within the text itself. At one point Jubal is justifying his own writing, and he states "I never hide from him in a private language, nor am I seeking praise from other writers for "technique' or other balderdash" (310). In other words, maybe Heinlein is saying that he has a point he wants to get across, and he's going to do so no matter the criticism he might get for lack artistry in the process. Of course, this doesn't change the fact that it's simply not entertaining, but perhaps it was intentional.
One thing I will say, though, is that the subject matter must have been pretty surprising considering the book was originally published in 1961. I can't imagine the American audience at large was very receptive to something like this, and that in and of itself is pretty fascinating to think about.
If you can't tell, I would definitely not recommend this book, unless you are willing to stop after the first two parts, and not even bother with the ending.