Shermer is a decent writer and a very sharp thinker. I'm in basic agreement with much of his worldview, although I think he's not skeptical enough here and there. He set himself several tough tasks in this book, and I'm not sure he really succeeded at any of them. Yet I don't think he really tried either, his main point was different.
He didn't go into enough depth about the evolution of altruism or cooperation. For that, I suggest you turn to Matt Ridley. He didn't go into enough depth about free-will either, but that doesn't matter to me since I think it's probably an insoluble problem. He does a very good job of covering the pop-culture level of debate on ethics, but I think he should have explored various philosophical positions much more thoroughly, and I would have been very pleased if he had covered the ethical positions that various skeptics have held in the past. His own provisional system of ethics are as reasonable as any other, although his attempt to label them scientific is dubious.
So I guess the point of this book was to engage in the pop-culture debate on ethics, to take on Dr. Laura and the religious right. So he avoided philosophical complications and so on, trying to stay relevant to America in the 21st century. Actually I'm not sure how to go about that project, but I appreciate the attempt.
The book was pleasant reading, and I enjoyed it. I'm sure that there are deeper, more thorough coverages of everything in it, but probably few are so easy to read. If you're new to the idea that a non-religious worldview could be supremely moral, this is a book that will suprise you; if that idea is old news to you, this book will entertain you.
I'd like to add that I think some of Shermer's other work, especially "How We Believe," is much better. I'd recommend reading that before this one.