It's a good read, entertaining, and except for the ending it stands on its own nicely. The focus is handed off back and forth between Jane, the love interest from the first book and a new character, Jared, the clone of a mad scientist who has been grown to be a Special Forces soldier. Jane is Jared's squad leader, but because he's been cloned for a nefarious purpose by the top brass, he also does time as a guinea pig.
I have an observation which others may look at as criticism, but I'm fairly neutral about because I tend to do the same things when I write fiction. Scalzi is very light on physical descriptions of his characters. He has painted himself into something of a corner by making his Good Guys look a lot alike (green skin, cat-like eyes) but hey, these folks are all gen-gineered clones from real people, they ought to have some identifiable differences other than personality. The several non-human races also get shortchanged this way - I have no clear image of what any of them look like.
He is also very light on physical descriptions of the settings. He only mentions items in a room if they are being used by the characters, and he only mentions features of a place in the most general terms. If you're a fan of those books which have maps in them, you will find the tales of the Colonial Defense Forces to be lacking in this regard. Artists would have a difficult time illustrating these books. In fact, the covers for both books are pretty generic.
Both books are all about the minds of the characters, which brings me to a non-neutral criticism. The Jane of Ghost is not the Jane of Old Man's. They have different personalities, and more important to me, she never thinks about or communicates with the protagonist of the first book until the final pages, when the last chapter of the first book suddenly becomes the focus for the ending of the second.
Scalzi has created an interesting universe, and sets up the framework for an infinite number of spin-off books. I'll be reading the next one, The Last Colony.