"Old Man's War" was a long time in my "to read" list. Today I finally finished reading it.
I've been following Scalzi for some time on Twitter and he always post intelligent comments. I read Subterranean Scalzi Super Bundle and really enjoyed his writing style. The very first book of his was, then, an obvious choice.
"Old Man's War" is part The Forever War. It's war on space. You have spaceships and soldiers with special training and special weapons.
But while you read about the military life like in "Starship Troopers", it also has no qualms in killing interesting characters to show that not all things in war are easy and the bad guys always lose, like in "All Quiet".
The story starts really strong, gets more cerebral in the middle and then slows a bit in the end. Still pretty enjoyable, specially if you liked the books I mentioned before. The fact that they way Scalzi writes makes it easy to read is also a plus.