Scalzi is a new author for me and I quite like him. If I'm remembering correctly this book was originally offered as a serial on his blog I believe and got enough good word of mouth that a publisher picked it up. Very cool.
In the acknowledgements he says he was influenced by Heinlein quite a bit which is fairly obvious in reading the book. And he uses sci-fi author names within the story as place/people/thing names which is kinda fun. Like a little game of "spot the reference."
So, the general idea is that there are colonies on other planets that need to be protected. But the colonies are doing all they can just to survive in hostile environments so they need a military force. Which is recruited from the retired populations of Earth. All the people know when they sign up is that they will somehow be made healthy/young/fit/something and will be able to be soldiers. Of course it isn't quite as simple as they all think, but you knew that already.
There's a very similar tone to Starship Troopers and it's very much that these people are fighting for their buddies and not really for any government.
And then there's the Ghost Brigades... who weren't quite people to begin with and start out with adult bodies. So there's these elite soldiers that are emotionally toddlers. The interactions are rather interesting to say the least!
I quite enjoyed the writing style too, I can't quite figure out the exact word I want to describe it... kind of irreverant I guess. Not quite as formal maybe. I don't think I'd want everything I read to be written this way, but it's a nice change now and then.