The book owes an obvious debt that the author acknowledges to Heinlein. It is shorn of the depth of a Heinlein novel however, although its central premise is silly enough for a Heinlein novel. It imagines a future where at age 75 people join the military. People 75 and over I think would make the worst soldiers imaginable, but one must put this aside.
It really is just an ingenious military soap opera. From time to time Scalzi pretends that there is more going on, but there isn't really. It's often compared to Heinlien and to the Forever War, but it really cannot bare the weight of that, and it is best to read it as an escapist space opera and nothing more. If you take it on those terms its fine. Does not deserve its stellar reputation, however.