I've got to admit that I'm not a great fan of Dawkins: partially because I'm not drawn terribly toward popular science, but mainly because he always comes across as rather conceited. I think the real struggle is not between religion and atheism but moderation and extremism, and sometimes Dawkins sounds a little too strident for my tastes.
The reason I disliked Black Mass was because it seemed like an angry jumble of thoughts. I couldn't really wring any sustaining thread out of it except "Everything you think is right is wrong, and the world is ending". It seemed to be missing a central argument, and certainly didn't make any sort of useful suggestions, which is what I'd expect (perhaps unfairly) from a polemic. Gray seemed so pleased that the Western world is about to be annihilated (serve you right for Iraq, horrid Tony Blair!) that I began to suspect that he didn't believe it really was going to happen at all.
In fairness, I should add that I don't share Gray's politics - in as much as anything, I tend vaguely towards the somewhat defunct Euston Manifesto - and I felt that once the rhetoric was done, his argument stopped at "The Iraq war was illegal and Bush is a religious crank". I wanted to ask him "Yes indeed, and now what?". That said, had he been making an argument I liked or agreed with, I'd probably be much more friendly towards the book.