Technologically, this book is a bit dated, obviously (slide rules? books full of numerical tables that a modern cell phone could store in 1% of 1% of its memory?). But as a yarn, it's great, worthy of the Dean of American Science Fiction. I'd rate this towards the top of his "lesser" novels, up there with "Citizen of the Galaxy."
This is one of Heinlein's so called children's books, however I never read them until I was over twenty and still enjoyed them thoroughly. While they are not as philosophically deep or as textually complex as books such as _Stranger in a Strange Land_ or _The Number of the Beast_, they are wonderful escapes that can be consumed in an afternoon or two.