Perhaps a 3.5? Good stuff, solid, intelligent and exciting, with interesting concepts of the evolution of a species and technology. Just, for some reason, it wasn't quite as good as Old Man's War (which was the best of Scalzi's so far) or Android's Dream. 
 Scalzi's characters were more convincing (and funny) as old people turning into young bodied super soldiers than babies being grown in an adult body. For one thing, I wasn't convinced that these super soldiers were 1 year old or less. They talked and sounded like adults to me. The antagonist's motives didn't quite convince me either, the young daughter who's minor role ended up being somewhat important was an annoyance with her baby talk, and lastly, the ending was a tearjerker that was slightly too pat. 
 Still, it was entertaining and worthwhile.