Actually, I picked it up not too long ago, looking for a good suppliment that figure out some of the campaign-impacting aspects of war.
I am currently, in my home game, running a game based out of Durrover in the Scarred Lands, which is at war with Calastia, and trying desperately to avoid being over run and absorbed into the Hegemony.
This is actually a really cool book; I have to admit to enjoying it quite a bit. There are actually some really good feats in here that I am allowing in my campaign (something of a rarity for me), and the prestige classes are, in my opinion, nicely balanced, even if they are a touch on the generic side (something necessitated by the fact that War is meant to be used by anyone; its just my own personal preferences lie in the other direction).
The sections on warfare, including the system looks fairly interesting, though I haven't had the chance to playtest it yet. I'm actually thinking of setting up a scenario or so and letting my players take the part of the Durroverian resistance forces against the Calastian war machine, just to press home how dire things are for the small Nation Between the Mountains.