Don't get discouraged if you sense you're not getting the most out of it because you can't identify too many of its connections to other books: read as many of Heinlein's books as possible (start by the ones mentioned in the other favorable reviews) and you'll see that Heinlein's characters and plots aren't confined to a single book, indeed they exist well beyond the written lines of each of his books; by this I don't mean just the intertwining characters and realities he frequently comes back to, but rather his recurring and easily recognizable views on human nature...
If you can't see the beauty in how the "last part" of the book detaches itself and therefore reads so differently from the "first part", then you haven't the faintest idea of how to fully appreciate the gift Heinlein has given us with this book...
Two years ago I didn't like it because I wanted a simple, happy adventure story. I didn't want to have to deal with the implications of the second half of the book. Since then, I've read, and *forgotten* a lot of easy adventure books. Yet this book has always stayed in my mind. I haven't yet read many of Heinlein's works, but now I'm going to start. And the ending? You'll never forget it!