This is a great Sci-Fi/Military book in the vein of Robert Heinlein's Starship Troopers. I got it in the mail yesterday, I finished it today. I couldn't put it down, it was that good.
The basic premise is that in the future, old people sign up for the Colonial Defense Force so that they can get a chance to be young again. They don't know anything about what is out in space, since the Colonial Union keeps it from the people of Earth, but they do know two things about signing up: They can't come back to Earth, ever, and their bodies will somehow be fixed so that they can fight for the CDF.
Old Man's War is filled with interesting sci-fi premises, and it has its share of action as well. I'd recommend this to anyone who likes sci-fi, it was a fantastic read. I'm looking forward to reading the sequels, just as soon as I finish I Am Legend.