Dawkins carefully presents the preponderance of evidence supporting evolution from diverse disciplines: molecular biology, paleontology, geology, chemistry, etc. It's a wonderful book for people who don't often think about evolution; it weaves together the interesting stories from all these areas of research in a way that inspires awe at the complexity of life, and amazement at the simple theory that fits all the pieces together.
On the other hand, as another reviewer, Chet Raymo, pointed out, there's not a hell of a lot that a well informed reader wouldn't know (especially if he or she had read Dawkins' previous tomes). This begs the question:
In fact, I dismissed as overblown Dawkins' insistence that we need to convince anyone of the reality of evolution, with the exception of schoolteachers and scientists. I think that most people who choose not to believe in evolution are doing so not because they've carefully weighed the facts and been persuaded by creationism, but rather because it's part of their religious upbringing and they choose not to believe in evolution (which is, after all, their right).
To whom, then, is it addressed? Presumably to the fair minded but not so well informed person who has heard a lot about the evolution wars and wants to know just why scientists are so confident that they are right. If this is the case, I would have thought it best if Dawkins had left out the snarky and condescending comments about creationists. Admittedly, they are easy targets, and perhaps Dawkins by temperament cannot resist taking pot shots (I do it myself), but the book would work best, I believe, as a persuader of the unpersuaded if he had let the evidence stand on its own, untainted by polemic.
I'm slightly less confident of this assertion after seeing craziness like this 50 page introduction to On the Origin of Species by evangelist nutjob Ray Comfort and pimped by Kirk Cameron (of all people). Comfort presents seductive, but completely false rebuttals to many of the topics discussed in The Greatest Show on Earth (as well as throwing in random quotations by Adolf Hitler). I can see how a sensible person could be convinced by his arguments, even though when viewed from an informed perspective one can immediately see that they are designed to seem plausible without actually being supported by any scientific facts.
Still, at the end of the day I don't really care very much whether the average American believes in evolution or not, as long as it doesn't extend to the active suppression of scientific thought. Raymo is right that Dawkins is preaching to the choir in a serious way here. If his goal is to change minds he needs to be less of a blowhard about his topic.