The damage that has been done to the world by faith-based reasoning and organised religion is covered in a fair amount of detail with plenty of anecdotal stories thrown in to give the reader something to latch onto on an emotional level. The supression of scientific development, the teaching of ignorance at the expense of scientific fact, the subjugation of women, the abuse and brainwashing of children, the fallacy that only faith has a moral code, and of course the jihads and religious wars are all covered.
The philosophical arguments in favour of the God Hypothesis, as he calls it, are presented and knocked down one by one in incisive detail, and many are exposed as circular reasoning. The foundations of religion, the ancient scriptures, are examined closely, and the cruelty and violence of the old testament God is explained in shocking clarity. God is portrayed as an insecure, vengeful, jealous, genocidal maniac. Interestingly, there are examples of scripture that look suspiciously like they were duplicated during centuries of editing. For example, Lot's departure from Sodom (after he showed God how righteous he was by offering his daughters up to a mob to be raped in order to protect some angels who came to visit, Genesis 19:7-8) is suspiciously similar to another story in Judges 19:23-4. The mysoginist tone of the monotheistic holy books is made very clear throughout all of this, indicating how it was all written by men in a time when women were considered almost sub-human, and there are people in the world today who still want to adhere to such inhumane principles both in Christendom as well as the Islamic world.
There is an interesting exploration of the Catholic church's talent for making things up as it goes along using its own self interest as a motive and warped reasoning as the justification. Purgotory was invented as a means of justifying prayer (also exposed as ineffective elsewhere in the book) for the dead and as a means of generating revenue in the form of 'indulgences,' something the author describes as 'the medeival equivalent of the Nigerian internet scam.'
At all times, Dawkins keeps yanking the reader back to the reality of how the world works as explained by science. Natural selection is described not as an unlikely 'chance mutation' but as a gradual process in which the development of complex organisms is actually inevitable in many places in the universe wherever the conditions are suitable. Intelligent design advocates look for holes in scientific knowledge that can only be plugged by a god, only to retract whenever science plugs the gap with new discoveries.
Some people deride atheism as an 'empty existence' and having 'no meaning.' Dawkins extolls the virtues of atheism coupled with a deep understanding of the wonders of science. The universe as known to science is much bigger and much more impressive than the relatively small and young concoction of the ancient peoples who needed simple stories for simple people to explain that which they did not yet have the scientific knowledge to understand.
If you are an atheist and have been shy about voicing your beliefs (or lack thereof) in the presence of religious people, reading this book will give you new confidence and pride in your preference for reason over faith.