John Scalzi self-professes his two top influences to be Robert Heinlein and Dorothy Parker. As soon as you read "Old Man's War," you'll see plenty of both, to excellent (and of course pretty darn funny) results.
Cliched as it is, I could not put this book down. This book has an average rating of more than four stars on here and was nominated for the Hugo. It's easy to see why. I can't think of a single thing about this book I disliked. I could be more easily pleased or optimistic than the average sci-fi reader because I typically stick more to fantasy than SF. Perhaps the story of a 75-year-old going off to interstellar war is more trite than I am aware of. Some reviewers call it "Ender's Game" without the kids, but I think that description falls way short. Yes, it's military science fiction. But it's John Scalzi's first novel, and am amazing one at that. The writing, characters, pace and storyline are all spot-on. I laughed, I cried, I couldn't wait to see what happened next. I was surprised at the twist at the end. The pace is pretty fast -- there are some substantial topics Scalzi skims the surface of, but I don't fault him for it: there are four books in the series so far, with another one the way later this year. Perhaps he'll address the ethics of colonization or use of soldiers farther down the line, and perhaps not. None of that detracts from a thrilling, fun, rapid read. Thanks to "Old Man's War," I will definitely be checking out Heinlein, and more of Scalzi, in the near future.
If you're interested, Scalzi keeps an equally awesome blog, here: whatever.scalzi.com. This is my favorite post, on "Who Gets to be a Geek?"