I just got Fields O' Blood yesterday, and already I think it rules. It kicks @$$. Eden studios put out some errata on it already, which can be found on their website, but it really isn't much compared to the amount of material that is packed into this ~175 page book.
Just leafing through it, I am impressed with the PCs being rulers, how magic is integrated into mass combat, and seige weapons. The system is easy, and from what I read it can be broken down to smaller units if needed, or just keep them as a few large swarms that can be used for quicker-run battles. There are rules for laying seige, figuring out fortifications and time/money required to build them for a PC kingdom. This is pretty much what I have been waiting for, as players get bored with saving someone else's kingdom; they become much for enthusiastic when it's their own they have to worry about. That and quelling riots, taxing fairly, dealing with disloyal barons, war, spies, bribing dragons to stay away... nearly everything.
The book has a chapter committed to transitioning the PCs into rules instead of just saying "the king died, and left you his throne". There are a few feats and a number of PrCs, both of which are dedicated to large scale war, but that is not the focus of this book. If you want more PrCs, feats, classes and such, go get Complete Warrior. If you want kingdoms designed for the PCs to run them or against them, or want to have the PCs in charge of a large army, this is THE book.
I was waiting for the longest time for the book to come out, I thought my expectations might be too high, but then it covers everything I thought it would and more. Though Cry Havoc is...s'ok, I would buy this book first.
And no, I don't work for Eden Studios, nor did I play test it, nor did I get a free copy. I just like the book, and will read it cover to cover. Maybe even twice.