One of Heinlein's early juveniles, this one has all the elements seen throughout his juvenile series: a plucky boy hero who's always wanted to go to space, precocious girl heroine (who fortunately is too young to be mooning over boys), Father Knows Best who turns out to be a hidden genius and former Very Important Person in the government, and interesting 50ish aliens.
The thing I like about Heinlein's juveniles is that they still hold up pretty well 50 years later, if you can ignore all the references to slide rules. The entertaining quality of this book made it a great listen (I listened to the Full Cast Audio version which used different voice actors for different parts, and sound effects for all the alien voices) and the story just hums along. It manages to retain the "hard SF" feel of most of Heinlein's early work despite the surprising turn the story takes when what seems to be another "space camp gone amiss" adventure ends up becoming literally intergalactic in scope.
Heinlein was writing to be paid, not to make his mark on the genre, so there isn't much here to challenge your expectations. The author does slip a bit of his slightly smug philosophy into the pages, but fortunately this was before he went Full-Tilt Libertarian. It's Boy's Adventure for Boys, but the character of Peewee - a spunky genius 10-year-old - makes it something a girl could enjoy too. She isn't as active a protagonist as Kip, but she does do things and figure things out on her own and generally acts like a super-bright super-perky thorn in Kip's side.
Basically, this is just a helluva fun book, and a good one to read for any sci-fi fan, whether you are a Heinlein virgin or a long-time fan.