My main criticism of the book, as of Dawkins', is that it attempts to do too much, and as a result can seem scatter-gun, rather than perfectly aimed, in its assault on its targets. The recent crop of atheist polemics seem to want to do a number of things at once (and in 300 pages): to prove the non-existence of God (or at least the unlikelihood of his/her/its existence), to expose the mythical or fabricated nature of much accepted religious 'truth', and to demonstrate that religion is bad for us, either as individuals or as societies - when any one of these would be sufficient ambition for a single book.
Both books tend to be stronger on the first two aims than on the third, with Dawkins as a scientist clearly in his element in showing how evolutionary theory has made the hypothesis of a divine creator unnecessary, and Hitchens with his background in literature and the humanities ably demonstrating the man-made character and historical development of many religious ideas. But my sense is that attempting to prove that religion is bad for people is a battle that can never be won: believers will always counter the atheists' catalogue of religious abuses with alternative instances of saintly individuals, or religion's involvement in progressive causes. Moving the debate on to this territory gives religionists the excuse to avoid responding to the much tougher questions in the other two areas: and indeed, much of the religious reaction to Dawkins and Hitchens has been in these terms - trying to show that it was, after all, Martin Luther King's Christianity that inspired his anti-racism, or that slavery would never have been abolished without religiously-inspired campaigns. (I've written elsewhere about my annoyance at the way that much contemporary argument in favour of religion is cast in these utilitarian terms - attempting to show that religion is a 'good thing', rather than to demonstrate its truthfulness or believability.)
As a former believer who wavers between fascination with faith, day-to-day agnosticism, and occasional outright unbelief, I think that it's the second area of contention that should be the main focus of debate. Scientists will never be able to prove conclusively the non-existence of God, any more than believers can prove his existence. But contemporary believers need to respond to the critique of religious ideas as man-made and historically contextual, of the kind put forth by Hitchens and made possible by modern methods of literary and historical investigation. Do modern Christians agree with Hitchens, and most impartial commentators, that many of the characters and events of the Old Testament/Jewish bible are legendary rather than historical? And would they also concede that many of the Gospel stories about Jesus were added long after his death, often to support a theological argument, and that much of what passes for received Christian doctrine is the result of all-too-human faction fighting in the early Church rather than divine revelation? And having accepted as much, will they tell us exactly what they still find believable in the bible, and on what grounds?