I somehow forgot to post this review when I wrote it, so here it is:
A much easier read than I expected, Scalzi's Old Man's War is also an entertaining one. The author states in the acknowledgements that the book owes a lot to Heinlein (Starship Troopers) and I think you could probably add Haldeman's The Forever War to the list of inspirations and influences. As such, I didn't find the ideas to be particularly original but Scalzi's main character pulls the premise up and gets it off the ground in a way that makes the journey a fun and emotional ride. The story is focused on the main character, John Perry, a 75 year old who leaves Earth for the promise of rejuvenation at the cost of a soldier's life, and never strays. So we only hear about things that are important to the plot, or to Perry's development. The result is never really getting a very clear picture of what the universe away from Earth is like, which works within the framework of the storyline, but always leaves the reader wanting a little more information.
One concept I quite liked was the idea behind the Ghost Brigades, and I wish there had been more focus on the dichotomy of the CDF military: the titular “old man” infantry in contrast with the Special Forces “child soldiers.” Scalzi's next book is called The Ghost Brigades, so maybe I'll get what I was looking for in that volume.
The thing that frustrated me the most was the author's use of dialogue. It reminded me so much of Orson Scott Card's use of back-and-forth too-clever banter that quickly becomes nauseating. Now, it's not nearly on the stomach-upsetting level that Card has achieved, but I could envision a time when Scalzi gets there unless he reigns it in. I really hope it doesn't continue on this path, because his writing is otherwise very pleasant.
I'm looking forward to checking out the other books set in this universe, and maybe getting a better sense of the structure of the colonies.