"What is good and what is not good? Do we need anyone to tell us these things?"
It's a pattern as old as Socrates: pick a nice, empty rhetorical container on which there is some vaguely general agreement--"Good" or "True" or, in Parfit's case, "Reason(s)"--then fill it with whatever you want. It cannot be denied that Parfit has constructed a meticulous rhetorical edifice, a recitation of definitions to rival Webster, but the sum total of his contribution appears to be the claim that deontology, consequentialism, and contractualism should all yield the same ethical results (even though, as presently understood by their various proponents, they don't). All of this is founded on a notion of "reasons" that sometimes succeeds and sometimes fails, at least to the extent that an ordinary-language descriptive-ethics is what Parfit intends. Kudos to Parfit for constructing a work as labyrinthine as anything Kant ever penned; simply attempting to decipher some of Parfit's claims should give philosophy professors something to argue about at great length and for years to come (indeed, some such arguments are already included in Volume 2!). So good news, I suppose, if you're in need of a thesis, but do not expect anything approaching conceptual clarity to arise from this mess.
Somewhat more particularly, I felt that Parfit's most egregious mistake was classifying virtue ethics, without comment, as a sub-category of consequentialism (it's not). He completely ignores some of the most interesting ordinary-language ethics to come out of academic philosophy in the last thirty years. But perhaps he can be forgiven; what else is he supposed to do with a school of ethics that would tell him (rejecting the finality, as it does, of both consequentialism and deontology) that his magnum opus makes a moot point? As someone who leans toward virtue theory, this was a huge disappointment to me and the reason I'm giving the book one star instead of two.
I understand that Parfit was a philosophical rock star before I started grade school. And I am somewhat sympathetic with his stated aims--in short, he wants an ethics with the certainty and finality and universality of a naturalistic framework while eschewing all that metaphysical baggage. But as near as I can tell, the praise flowing toward this book amounts to the warbling of sycophants and hangers-on looking for some coattails to ride to tenure, or professors who do not wish to have their wisdom questioned by daring to dismiss so voluminous a work from so influential a scholar. I found very little to like about this book and much to roll my eyes at (not least of all, the utterly pompous tone with which Parfit dismisses possible counterexamples). If you happen to be an academic philosopher doing work in ethics you may not have any choice but to familiarize yourself with this book, but regarding its intrinsic value I wouldn't even recommend it to philosophy undergrads, much less layfolk.