While the title of the book is 'The God Delusion' Dawkins doesn't actually argue against the existence of God all that much, he uses the 'But who made God?" argument a few times, but generally he seems to be arguing against religions, rather than God. Indeed, he actually only says that there is 'almost certainly' no God, and never makes arrogant claims that suggest he actually knows.
But what he does argue against a lot is both religion and creationism. He argues, quite well in my opinion, that religious thinking is very bad for science as it stops people question and keeps them content with faith, and also things it immoral that children are indoctrinated into the same religion as their parents when, he says, a child cannot really understand religions and should be allowed to make the decision at an older point in their life (which is also very convincing). He also talks about the extent to which atheists are prejudiced in America, which is actually pretty surprising once you read it. Plus, as a kind of little bonus, he goes into the crazy world of quantum mechanics at the end.
So, on the whole, this book is fairly argued and well written. There are several funny little jokes and anecdotes that he slips in every now and then and he makes several pop culture references which make it a little more enjoyable. The downside is that, as Richard Dawkins is a biologist, the tone becomes a little too scientific and hard to understand when he is talking about evolution, and, as I said before, the book does not really fully fulfil its task; he seems to argue well against religion and creationism but not so well against the idea of God itself. So on the whole, I'd rate this book as an 8/10.
(You can buy it here: http://www.amazon.co.uk/God-Delusion-Richard-Dawkins/dp/055277331X/ref=sr_1_1?ie=UTF8&qid=1312215559&sr=8-1)