Let's start with the positive.
1) This book presents a look at how moral theories work from outside of them-that is, it tries to describe them without their various assumptions. I can't say that the book is worth it for this reason alone, but it does make one contemplate moral theories from a different perspective.
2) This book is a comprehensive survey of the status of moral theories. Williams makes a strong (and largely successful) attempt at presenting the current state of the most important current (as of the writing) moral systems. There is also a criticism of each different system. (As the other reviews will tell you, Williams is opposed to the very idea of morality, if not ethics)
Now for the negatives.
1) Williams consistently misreads or misrepresents the philosophers he is criticising. This is perhaps the most important fault of the book, as the purpose of the book is a critical evaluation of morality, and that can scarcely be convincing or useful when the opposition is not accurately rpresented. The most glaring example that comes immediately to mind is his account of Kant's "psychological hedonism"
2) His writing is simply sub par for any serious work of philosophy. I spend more time deciphering his sentences and paragraphs than I do with those of Kant, and not because they are dealing with complex and difficult subjects, but because they are meandering and poorly organised. It is somewhat rare to find a paragraph in which he sticks to the subject at hand.
3) Related to #2 is the lack of argumentation in the book. Williams often makes sweeping and brazen assertions without one shred of argumentation, especially in his account of human moral psychology in the chapter dealing with Kant and in his conclusion in the last chapter. Such argumentation is what, ultimately, separates philosophy from mere dogma, and is conspicuously absent in much of Williams' work.
4) The last problem is with his philosophy itself. He purports to remove morality from ethics, but leaves no account of what ethics is without morality. The best description that we get in the book of ethics without morality is that it is "vague" (p. 6) and that ethical considerations are merely one type of practical consideration. (p. 182) In the final analyss, Williams tells us that we should act how we want to act because some of our desires are more important than ethical considerations, but each agent must decide for himself, without any guidance from anything but teh agent's own dispositions, which of his desires are able to override ethics, or whatever is left of ethics after Williams is done with it. In fact, what would even count as an ethical consideration is unclear.
This book is largely unsatisfying. Why should I slog through 200 pages of sub-par philosophy to find an ethical account less sophisticated than that of most two year olds? At least two year olds are clear in their reasoning. The reason that I even gave this book 3 stars is that it is important to know ones enemy and Williams' perspective on morality, if not ethics, is rather interesting. Finally, I must say that, as much as I disagree with the other reviewers, one is absolutely correct in recommending "Internal and External Reasons." That paper is everything that this book isn't, as well as being Williams' most important contribution to moral philosophy. It is also quite crucial in gaining a detailed understanding of Williams' positions in this book.