There. Now that that is out of the way, John Scalzi's Old Man's War is a gripping, enjoyable military science-fiction novel whose sole major fault is that it feels too damnably short.
The title is unfailingly accurate — it is the story of an old man's war. Join the Army, see the Galaxy, meet exotic aliens... and, well, kill them. Seventy-five year old widower John Perry takes a second shot at life and abandons Earth to enlist in the Colonial Defense Forces. Conveniently dropped into a new enhanced body, Perry is soon hip-deep in a decidedly Darwinian conflict between various sentient species for habitable planets. The story arc follows the predictable set-up — boot camp (of a sort), first combat, the progressive hardening of the character through more varied combat experience.
Though the story arc is a familiar one, Scalzi has peppered his work with an abundance of nifty concepts, social commentary, technology and some interesting twists on the usual aliens, as well as tight writing and good characterization (and a slightly sick and off-beat sense of humor).
Aside from the story length, the only quibble I had plotwise with Old Man's War was the black-and-white, everybody-versus-everybody vision of the OMW's universe. This reads as a bit shallow and unbelievable — not because I couldn't believe in a Darwinian universe, but because I would have expected that the conflict would not be quite as simplistic as it seems portrayed here. I would have expected more alien species using different evolutionary tracks to succeed (i.e. some parasitical, etc.) rather than just straight-forward violent competition. But hey, that's me.
You may now resume your regular programming.
If you are looking for more military science fiction to read, check out the Baen Free Library, courtesy of Baen Books (Note: not the publisher of OMW — that would be Tor Books) . The Baen Free Library includes works from such authors as David Drake, John Ringo, and David Weber (for the excellent Honor Harrington Series). I also recommend the old classic Starship Troopers by Robert A. Heinlein if you haven't read it yet. (Note: I've linked to the version with the classic cover, not the trashy movie cover version.) And as a bonus, I recommend my personal favorite Heinlein, The Moon is a Harsh Mistress.