It's a good book, not a great book. Whilst I'm very sympathetic to his views and am certainly an atheist, the problem is not so much that it is incredibly polemic, but that it's polemic to the extent of being either deliberately misleading or just plain wrong on certain points.
Where he's good, he's really very good, particularly (obviously) on genetics and biology. On the pyschology and social anthropology he's, self confessedly, good but doesn't explain everything (his explanation of 'meme' theory is frankly woeful). On theological history he's accurate but very selective in his choices of data to say the least (his description of the Council of Nicea is, shall we say, slightly partisan). On certain other area, for example, the impact of religion on international conflicts, he's laughably bad - his claim, amongst others, that religion is responsible for almost all the major international and civil wars in the 20th century is just so wrong (and such a foot stamping petulant teenage analysis) it's untrue.
Don't get me wrong, I like his view, his style of writing is attractive and he makes his case in a very compelling way, but just don't take it all at face value.