Old Man's War is his first, from 2004; it was a Hugo finalist and got a lot of critical acclaim. It's evident from the beginning page that he's followed in the footsteps of Joe Haldeman, Paul Verhoeven, and Spider Robinson, choosing to write a novel that is his own take on Heinlein's 1959 classic Hugo winner, Starship Troopers. Now if you've never read Starship Troopers, you're missing out and need to start there. (If you've never read Haldeman's The Forever War, on the other hand, count your blessings.) All these works start with the premises that humanity needs to expand off Earth; that aliens will oppose these human endeavors; and that humans who "ain't a-gonna study war no more" are going to get wiped right off the annals of history. There are varying takes on why a volunteer army is necessary and how and why it would be constructed, as well.
But that's where the similarity ends. Heinlein's novel, like most of his novels, is a thinly disguised prolegomena to the metaphysics of morals; although his characters and settings are believable and interesting, he gets a little preachy at times. The preaching is good enough that I can still get into a good argument with a fellow SF fan over his points, 50 years later; that's good literature by anyone's definition.
Haldeman's work, on the other hand, adds little new to Heinlein's; I felt like Haldeman saw his duty to update the classic text for a young audience, but I didn't think he added much.
However, I really liked Old Man's War, and that's why I'm bothering telling you this. Scalzi's a sort of shoddy prose stylist - "garrulous old fart" is an example of a phrase he uses, mixing a 50-cent SAT word with a rather annoying phrase that recalls middle-class white biker retirees. The entire novel sort of suffers from this; you get the idea that all human spacefarers are middle-class white Midwesterners from the 1950s who are updated with a profanity plug-in.
But Scalzi has a good sense of character, a competent sense of narrative arc - the book really moves through its three acts, you never feel like he's getting lost - and he brings a sort of Larry Niven sensibility to his xenobiology; the alien races are plausible, interesting, fun to read about. He has a good sense for SF magic and he writes it about as well as Heinlein did - in fact, at one point, he has a pod bay door "iris open," which is a not-so-subtle nod to a famous line from Beyond This Horizon (in which, at one point "The door dilated." is thrown in without any explanation or context; the line was dissected by Samuel R. Delany in a well-known essay.)
You also never feel like you're getting a lecture on epistemology from Scalzi. He is writing an SF romp through the galaxy. You get to suspend your disbelief and engage in a bit of fantasy. Scalzi's original idea - that the volunteer army comes from Medicare-age people who are taken off Earth at age 75 and rejuvenated, in return for their military service - sets up some fun plot points and Scalzi handles the idea deftly. It's fun.
I've been thinking a lot about the role fantasy used to play in my life; I've been missing it, lately. Everything lately is very real. Scalzi's book took me out of my own little life for a while and sent me on a tour of the galaxy. That's what good SF is supposed to do and I suggest you read it now.