Whether or not you believe that there is a god (God) guiding evolution, there is no rational argument against its existence and importance. Nor does an understanding of evolution diminish the wonders of the world- in fact, it augments them. Those are Dawkins' main themes in this book, and they are laid out clearly and forcefully.
The first main theme, the evidence for evolution, is the dominant theme of this book. Given his previous work on atheism, this certainly is a topic he believes passionately about. Dawkins is an authority on evolution, and has been ever since his first books on the topic (The Selfish Gene, The Extended Phenotype, and to a lesser extent, The Blind Watchmaker, are classics in evolutionary theory). What Dawkins does in this book is recite the mountains of evidence that support evolution. Fossils, development, cross-species similarities, and genetics all add up to make evolution one of the most solid and proven theories in science (perhaps more so than gravity!). He even goes the same route as Darwin in using domesticated animals as an example of evolution (guided by humans, not natural selection) in action. People who believe that the word "theory" means unproven, unsupported guessing really need to read this book to understand just how much evidence there truly is for this "theory". Dawkins even left out some of the more dramatic genetic evidence that has begun to surface recently, and the case is still rock-solid. Dawkins does avoid making direct attacks and comparisons to other "theories" such as Intelligent Design, but his case for evolution is so overwhelming it's hard to imagine a competitive theory that could knock evolution off. As a defense of evolution, this book is a hard to argue against.
The second theme, that evolution doesn't take away mystery, but rather adds to it, is less forcefully presented, but it is omnipresent throughout the book. I suppose a lot of that has to do with one's personal preference. If you like mysteries and not understanding things, then understanding evolution might not be wonderful to you. But if you are curious, then evolution's explanation of life on Earth makes all life that much more fantastic. And certainly, there are still far more questions than answers when it comes to our understanding of all life on Earth. Evolution is the best tool we have for answering those questions, and the many, many, many examples in this book are bound to fill a curious reader with wonder about how and why life has turned out the way it has.
Overall then, I find this to be Dawkins' best book since his first three. I would quibble that at times he is distracted with an antagonistic tone, but I suppose it must be frustrating constantly defending one of the most proven scientific theories ever. Ignoring that, his writing is clear and easy to read. What's more, the subject matter is so fascinating that even those familiar with biology and evolution will enjoy reading it. Dawkins is right on two things- Darwin's theory of evolution is 99.9999999999999999999999999999999999% likely to be true and the story of the evolution of life on Earth is truly The Greatest [Story] on Earth!