To start out, I am an atheist, and I believe in evolution. Even when I still was religious, I never really saw a conflict between the idea of God and science - I think it helps that I don't think I ever really took the Bible completely literally, especially when it came to the creation story - instead, I reconciled the two ideas with "God has a different concept of time, and evolution is how he intended things to happen." As a result, it has always been a surprise to me how many people argue against evolution because it goes against the Bible rather than any real factual reasons. Or why people would have a problem with teaching science in school, and religion at home. So in ways, you could say that Dawkins was preaching to the choir in this book, but unlike The God Delusion he isn't making an argument against God in this one, just an argument for evolution, and it's a good one. Even though I believe in evolution (it's science, it's fact, how could you not - actually I think the word "believe" is wrong here since evolution isn't a matter of faith - I don't believe that 1+1=2 nor do I believe in gravity; they simply are), I feel like sometimes I have forgotten all the exact arguments so it's good to read these type of books to refresh my memory so I can say more than simply "no one is saying we are descended from apes, only that we have a common ancestor."
For example, very early in the book, Dawkins reminded me of the definition of the word theory as it used in science. People like to argue against evolution because it still carries the title "theory of evolution" but I had forgotten that when the general population uses the word theory, they are actually using it the same way that scientists would use the word hypothesis. In science, theory has a much stronger meaning, and is as close as you can get to fact, though it could still theoretically be disproved - even gravity is still a theory in these terms, after all.
Dawkins begins his book with a simple discussion of natural selection and cites several rather recent experiments that show how species adapt to their surroundings as well as using artificial selection on the part of humans to show how certain traits can be bred for in agricultural crops and dog breeds, for example. There really isn't much to argue about when it comes to natural selection, and from here, Dawkins can branch out to wider topics, showing how natural selection would lead to the development of different species in different areas with the occasional mutation thrown in. He also cites several examples that disprove the idea of intelligent design that I found rather interesting. For example, there is a vein or muscle or something that goes from place in the neck to another - except it passes by it and loops back around. Dawkins argues that if this had been the result of intelligent design this would be a design flaw but with evolution it makes much more sense. Another example were testicles. At one point, testicles were inside the body, and at some point dropped outside (this didn't happen overnight). This is easily seen because the tubing connecting the testicles and the penis have a round about way of connecting instead of taking the straight route.
While he does occasionally harp on a point a bit longer than necessary, it really was a great book explaining all the scientific evidence behind evolution. As he explains, the fossils, which are some of the things creationists argue with the most, are really just the icing on the cake. Seeing how similar bone structures are across species, the similarity in DNA structure and how elegantly the idea of natural selection works are actually more than enough to prove evolution. If anything, the fossils give creationist a red herring to wave around, asking "where's the missing link?" In fact, every fossil is a link of some type.
One thing I have noticed is that whenever I read books like A Short History of Nearly Everything or The Greatest Show on Earth, I tend to get a little depressed - I realize that species have always gone extinct but reading these books about nature and the world just make it all the more obvious how much humans have screwed with the ecosystem, speeding up extinction for many species. It makes me wonder what would have happened if humans hadn't evolved the way they had and remained a part of nature, or if we had never evolved at all. As much as I hate to say this, it seems like maybe the world would have been better off. Still as long as one avoids existentialist thoughts, this is a great book to learn more about evolution and the scientific facts as well as a nice refresher for when one might need to argue one's stance.