There's nothing quite like an argument with fundamentalists to reinforce one's lack of belief. I found this several years ago when, as a vaguely noncommital agnostic, I stumbled upon the Yahoo! religion message boards; within a few weeks of fighting off frenzied evangelicals, my atheism was verging on the militant. This looks rather like Dawkins is having a similar reaction; as a world authority on the touchy subject of evolution, he probably gets considerably more religion-related abuse than most people, and now he's fighting back. Naturally, this is not a book that will appeal to many theists (despite Derren Brown's sneaky quote on the back cover practically double-daring them to read it) and that's a shame, as it contains a lot of good information.
The God Delusion starts off as less of a deliberate case against God* than a series of rebuttals to the usual theist arguments, which is then followed by an analysis of why religion is both unneccessary and actively harmful. The rebuttals are mostly superfluous if you don't actually believe in a god, but it's nice to see them all laid out so concisely, maybe as a handy reference in case any Jehovah's Witnesses happen to call. They generally follow the pattern of "Some theists say that X proves there is a God. However, X is actually caused by Y, not God, and here's evidence Z." It's no surprise to see that evolution comes up in many of these arguments - it is Dawkins's native subject, after all - but it was interesting to see him also using evolution as an active argument against the existence of a deity, alongside the usual (and comprehensive) trashing of Creationism and its bastard offspring, "Intelligent Design".
Having seen Dawkins in full ranting form on TV, I was a little worried that his confrontational style would be annoying, but he mostly manages to hold his frustration in check; there's only the occasional slip into emotive language, when you know he'd really like to be shouting "OF COURSE THERE ISN'T A MAGIC MAN IN THE SKY, YOU IDIOTS!". However, while he generally remains polite, he soon crosses into the dangerous territory of Religion versus Science. Many atheists avoid this, probably more from a desire for a quiet life than any well-considered ideology, but Dawkins pulls no punches. I'd previously been in the "quiet life" camp, but I now have to admit he certainly has a point.
Dawkins's erstwhile professional rival, Stephen Jay Gould, previously came up with the idea of "non-overlapping magisteria" - an unnecessarily technical term for the separation and peaceful co-existence of science and religion, where scientists handle the science stuff, and theologists handle the, er, moral and spiritual and non-scientific bits that are left. On the surface, that had always looked fine to me, but Dawkins highlights many flaws in the idea that make it effectively unworkable and incompatible with real science. Besides wondering what possible expertise theologians could have on any subject compared to, say, moral philosophers, he also objects to the very suggestion of "God" being used as an explanation for anything. This is not "outside" science, but actively opposed to it - it closes off avenues of enquiry and perpetuates wilful ignorance in the same way as would happen if we wrote off everything we didn't understand as "magic".
The perpetuation of ignorance is one of Dawkins's main arguments against religion, and is frankly a compelling one. The absence of need for any justification beyond "God says so" has led to many atrocities in the past and continues to do so today, and Dawkins argues that this is a direct result of religion's suppression of critical thought in favour of unquestioning devotion. The famous quote goes "without religion, good people would do good things and bad people would do bad things. But to get good people to do bad things you need religion" - to emphasise how a religious perspective skews judgement, he cites a chilling example of Israeli children approving of genocide when phrased in Old Testament terms, but being rightly horrified at it when the phrasing was changed slightly to make it a secular military matter.
The main targets for Dawkins's ire are the fundamentalists, mostly the ones of the three main monotheistic religions, and this has already caused many moderate theists to dismiss his argument as a strawman, not really applicable to their case. This rather misses the point, which is that all religions are fundamentally flawed by relying on something that doesn't actually exist, and is rather unfortunate, as it's likely that the moderates are the only theists likely to be reading this. They're also likely to be rather put off by Dawkins's occasional emotive outburst and his frequent comments about how much more intelligent atheists are - probably not the best tactic for a conversion job.
Most of the flaws here come from the author's personality showing through, not from the arguments themselves - while there are some great and funny moments, there are also some quite cringeworthy ones too, where Dawkins lashes out at his own personal critics, or tries to claim various notable historical figures as atheists while lambasting Christians for using the same tactic. Once his ire runs out towards the end, too, the book starts to waffle and I found myself skimming the last few pages rather than reading them. Still, while this may have been a case of preaching to the converted, one of Dawkins' stated aims for the book was to make atheists feel better about themselves - and whatever the committed theists may make of it, it's great for us unbelievers to have so eloquent a champion.
*or Allah, or Jehovah, or Yahweh, or Zeus...