This book title is well known and roundly criticized by various religions, but I think the majority of the criticize is not actually read it. It does not say anything specifically AGAINST any religion other than that which is already known. But I can see Christians, Muslims, and religious Jews in particular having a lot of trouble with it, because he points out aspects that are already known but rationalized away.
I admit that as a Reform Jew I do my own rationalizing away to make my own belief system "acceptable and reasonable" in some sense, but there you are…
As I say, this book doesn't actually introduce any new knowledge, but it puts it all in one place, and it points out a lot of religious fallacies and inconsistencies. Fortunately, I knew, going in, what I was in for, and the kaleidoscope of facts – although disheartening – is not overly surprising.
As a skeptic and scientist, I find the thought of religion overall to be more than a little suspect, and Mr. Hitchens puts it all together. I admit that I am not comfortable calling myself an atheist, and I definitely do consider myself Jewish, but that doesn't mean that the God nonsense is reasonable.