The central concept of John Scalzi's Old Man's War hit the sweet spot between originality and inevitability that makes for particularly resonant science fiction.
The Ghost Brigades isn't as original and doesn't seem as inevitable, which, early on, I took as two strikes against it. With Old Man's War the question was "Why haven't I seen this before?", but with The Ghost Brigades the question was "Where have I seen this before?" It felt more conventional and the "world building" of the earlier novel seemed to be replaced by "world elaboration".
But this is a case where my first impressions of a book really fail to do it justice. Though it isn't as immediately gripping as Old Man's War, The Ghost Brigades develops into a story that is just as thematically compelling and full of great neo-Heinlein nuts-and-bolts details.
But here's what I really want to talk about:
I recommended Old Man's War to a number of people who I knew weren't really "sci-fi fans". Like Starship Troopers or The Moon is a Harsh Mistress, it's a book that, IMO (based on reports back from friends), works pretty well for the non-fan. But even though The Ghost Brigades is, in the final analysis, as good a book (if not a better one), I don't think it has as wide an appeal.
How much should accessibility or universaility of appeal count for in terms of how "good" we think something is? If we're talking about most kinds of art/culture works, while we might look at universality of appeal as one measure of greatness, it's hardly ever the only, or even dominant, measure used and, in fact, the opposite law is often evoked (i.e. lowest common denominator criticism). But I do see the whole accessibility/universality stick getting pulled out quite a bit in internecine nerd debates about comic books, sci-fi books and movies, video games, role-playing games, etc. (Related: the phenomenon Jim Henley is talking about,here).