The book is a study in contrasts and does an excellent job capturing the juxtaposition of fractious, self-serving British tuatha versus the necessity to unite to throw off the bittersweet yoke of imperial occupation. It captures the flavor of the old gods (and their relationships to the various tuathas) versus the new god. It also explores the dichotomy of sustainable, guerrilla warfare versus the more "honorable" method of head-to-head battle.
I have to admit, some of the reviews are correct in that it takes a while to get going. Like 150 pages or so. Once Rome invades though, the book gets rolling and (for the most part) doesn't stop. It's a bit more psychological than martial, and a more assertive editor could have whittled it down a bit (it's nearly 700 pages), but it's a fascinating take on the period and highly recommended for those interested in the Roman occupation of Britain.