I really enjoyed this book. Mr Scalzi has rightly credited Robert Heinlein and there are a lot of similarities. But it is still a really good read. The realism of the battle scenes and the reality of classmates dyining across a stellar arena had very slight echoes of Joe Haldeman. Again, nothing wrong with that. Where Mr Scalzi wins out is the fast pace of the story and by rightly keeping mystery to the alien foes. So they never become humans in costumes.