I reread this one every so often, but it's hard to explain why. I think it's a little like the movie (in what is doubtless one of the ONLY similarities between the two) in that if you don't understand why it's good, there's no use trying to explain. You'd be hard pressed to find any other book so different from the movie, but I love them both. For different reasons, the movie wallows in it's own cheesy glory, and the book for reasons harder to pin down.
What struck me most about this book on this reading was how much I loved what it's not. It's not an action/adventure story. It's not a space opera. It hardly fits the bill of science fiction at all really except that Heinlein chose that route to go through what he wanted to go through. There is some technology stuff and some intergalactic travel, but it's really just a story about a guy who becomes a soldier in the simplest sense. It's about how he changes and how it happens. Mixed in are some ideas (which are quite contrasted with ideas Heinlein expressed in his other books) about how society should (or maybe just could) function. Oddly these parts (which amount to only a few pages of the book total) are the parts that generated all the controversy. People say the ideas are dangerous and resemble aspects of fascism. That's true enough I suppose, but it seems to me that getting caught up in that is missing the book for what it is. It's really not about soldiering or politics as much as it's about the fact that however much things change, they stay the same.
Rico's entire world is different than the one we live in. Its works different in terms of justice system, political approach and certainly technology, but that's not important next to the fact that it's a simple story about a man trying to find his place in the world, while struggling with the same sorts of the things people have always struggled with (particularly, though not just soldiers). A good read.