It's certainly a cathartic exercise for those of us who are frustrated by the amount of energy and agony wasted on belief in a mythical supreme being. So on the one hand, I applaud, laughing aloud, as Dawkins uses his acerbic wit to wonderful effect, mocking and ridiculing religion, holy books, and dogma. Every religion in the world would issue a fatwa based on the book, if any of them would read it or even take it seriously. But therein lies the problem: Dawkins is (to misappropriate a metaphor) preaching to the choir. His arguments against religion are glaringly obvious to someone like me; while I enjoy the way he says it, I don't need to be told that the Bible is a self-contradictory mess of unsavory behavior, that the Jehovah of the Old Testament is a brutal nutcase, or that creationists are ridiculous. And the people who do need to hear those things aren't going to read this book. Dawkins is mounting a very rational argument against religion, which rejects rationality and reason as bases upon which to understand the world. It's as pointless as a religious person telling a scientist that she simply needs to have faith. The book is amusing, shocking, worthy of guffaws and cheers, but ultimately pointless.
Perhaps the best section of the book is the one in which Dawkins returns to the subject he knows best, evolution. He opens Chapter 4 with Fred Hoyle's "747 argument," which says that the idea of human life arising by mere chance is no more likely than a hurricane blowing through a scrapyard and assembling a 747. Dawkins immediately dismisses the idea that we are forced into a binary choice between random chance and deliberate design, pointing out that evolution is littered with intermediate steps, failed experiments, and dead ends. Then he provides the obvious but unaswerable rejoinder to the argument that the world is too complicated not to have been designed by someone: in that case, who designed the designer?
But most importantly, he says that there's something cancerous about the religious side's habit of seizing on anything that science has not fully explained, as evidence of God. "It is an essential part of the scientific enterprise to admit ignorance," he says. "One of the truly bad effects of religion is that it teaches us that it is a virtue to be satisfied with not understanding."  Many things cannot easily be explained -- while he cannot explain how Penn and Teller do their magic tricks, he doesn't use that as evidence that "magic is real."
"I'd take the awe of understanding over the awe of ignorance any day," he quotes his deceased friend, Douglas Adams, as saying. An admirable sentiment, but have we learned anything here?
He also turns evolutionary theory to the question of why we have religion in the first place. It seems like natural selection should breed it out of the population; why do groups that waste time and resources on useless rituals and a parasitic priest class succeed more than groups that didn't do so? He comes up with an answer that makes a lot of sense to me: It's an unintended side effect of things that are evolutionarily valuable. Just as the enormous amount of time and energy we spend on sex is an unintended side-effect of the evolutionarily important reproductive drive. Unquestioning obedience, loyalty to a group, are all probably valuable evolutionary characteristics, but unfortunately they also lead to gullibility and infection by what Dawkins calls "mind viruses."
He spends an entire chapter on whether it's possible to have morality without God, but that's such a silly question that it's hardly worthy of his attention. Leave that to nutcases like Ivan Karamazov; like much of the book, it's a case of training intellectual howitzers on religious fleas. (And ignoring the philosophical artillery that has been focused on this question for centuries.) From there he moves on to demolish the "barking mad, as well as viciously unpleasant" Bible as a source of morality, digging up some episodes worse than any I remembered. (The case of Lot offering his daughters to be raped when the mob comes for the angels is a famously disgusting Bible story (Genesis 19:7-8) but at least those women escaped their fate. In Judges 19, another man does exactly the same thing, and the two women are raped and murdered and left dead on his doorstep. Of course, when the principal patriarch of three major world religions is famous for being willing to slaughter his son because a voice in his head told him to do so, what else is there to say? Dawkins says it for thirty or forty pages but to what effect?
I can't see this book making much of a difference to anyone. I came to atheism by two routes: increasing disgust at the hypocrisy and immoral positions of the Catholic church in which I was raised, and the realization that science and the natural world were more beautiful and inspiring than most of the Bible. It was the infectious delight in science of people like Isaac Asimov and Carl Sagan, rather than mockery, that led me away from religion. "I had no need of that hypothesis," LaPlace told Napoleon when asked why there was no mention of God in his work on celestial mechanics. God seemed no more sensible or real than the ones in my copy of Bullfinch's Mythology, while science was endlessly wondrous.
And again, while I think Dawkins is "extreme" only because the other side of the argument is, to borrow a phrase from him, "barking mad," he seems to believe that religion is the root of all evil. In this, he seems to leave his scientific approach behind; isn't it more likely that there's some inherent characteristic within people that drives them to divide up into groups and begin killing each other? Some unintended side-effect of traits that allowed us to fight and survive in the wild? If you magically removed religion from the world (or endorsed his truly frightening approach of removing children from their parents to be raised -- by whom? -- in a non-religious environment, arguing that the sexual abuse of children by priests was probably less harmful than raising the children Catholic in the first place) would the world really be any better? Yes, religion is ridiculous, and yes, horrible things have been done in its name, but people will inevitably do horrible things. "For good people to do evil things, it takes religion," he quotes Steven Weinberg as saying on page 249. Does he really believe that? Good people are not nearly as good as we'd like to believe and religion is more likely a symptom of that than a cause. Statements like "Without religion there would be no lables by which to decide whom to oppresss and whom to avenge."  are just silly.
In his "What's Wrong With Religion" chapter he has very little to say about ordinary religious people who do good things with their lives, instead focusing on dangerous fundamentalists. Yes, it's true that the 9/11 hijackers, or murderers of abortion doctors, are not "insane" but simply taking their religious books seriously, unlike the majority of their co-religionists, but why indict those who hold inconsistent, and therefore harmless, religious beliefs? Very few people manage to be completely rational in all of their lives, and if good people find good things in religious belief, more power to them. The battle is not against religion, but against intolerance and hatred, whether inspired by religion, political beliefs, or whatever.
The one thing I've taken away from this book is that I'll no longer waffle when asked what my religious beliefs are. He opens the book talking about the "religious" beliefs of people like Einstein, whose many references to God are largely metaphorical. Einstein described himself as a "religious nonbeliever" and his religion as the sense "that behind anything that can be experienced there is a something that our mind cannot grasp and whose beauty and sublimity reaches us only indirectly and as a feeble reflection."  I have often been reluctant to call myself an atheist because it seems reductionist and arrogant, to claim that one fully understands the universe and its possibilities. However, I do not believe in the supernatural; if there is a "God" then he, she, or it is just as subject to natural law as we are. We might not understand all of natural law, but I firmly do not believe there is anything outside of it. Dawkins' point is that my viewpoint is essentially a-theistic, in the sense that I do not believe in a personal "God" of any sort. ("Theism" is the belief in a personal God, while "Deism" is the belief that God is in no way still involved in workings of the universe.)
So yes, I'm an atheist, but I'd rather spend my time rejoicing in the wonders of the natural world than making fun of people who've found answers I don't agree with. A blistering attack on fundamentalism is long overdue, but this blistering attack on religion as a whole is a waste of energy.
Having read the book, I dug out a couple of articles about it: a New York Review Of Books piece that rednoodlealien sent me, and a surprisingly interesting article from the November issue of Wired. H. Allen Orr in the NYRB pretty much demolishes Dawkins' book, saying, "its arguments are those of any bright student who has thumbed through Bertrand Russel's more popular books and who has, horrified, watched videos of holy rollers." I suspect Dawkins ignores the details of thelogical arguments more out of contempt than ignorance, but Orr's points are well-taken. In particular, he quite rightly accuses Dawkins of comparing religion in practice to atheism in theory; he's right that it's unfair to lay the blame for the Inquisition and 9/11 at the feet of religion unless you also blame the Cultural Revolution and Stalin's purges on atheism.
In Wired, Gary Wolf suggests that Dawkins' crusade against religion (he also discusses Daniel Dennett and Sam Harris) is aimed at religious moderates; he quotes a London atheist who says, "Moderates give a power base to extremists." While it's certainly true that Catholics who use the pill and support abortion rights are allowing the Church to kill people in Africa with stupidity, is Dawkins' polemic going to change that? Harris' Letter to a Christian Nation might have a better chance, but liberal Catholics working for change would seem to be more effective than both of them put together. Wolf concludes,
Even those of us who sympathize intellectually have good reasons to wish that the New Atheists continue to seem absurd. If we reject their polemics, if we continue to have respectful conversations even about things we find ridiculous, that doesn't necessarily mean we've lost our convictions or our sanity.