It would be best to start out by saying that this book is, for all intents and purposes, a reprint of the Ravenloft Campaign Setting (henceforth known a the RCS), the Core Rulebook that originally updated the setting to the new Third Edition d20 rules. It brings all the mechanics presented in that publication in line with the 3.5 revision, but precious little (if any) new material has been added.
That being said, it is nevertheless just as excellent a product as its predecessor and does not deserve a low rating simply because a few excited DMs purchased it without first skimming the contents (This is not an insult - I myself was just such a DM!). I would recommend that any DM who already owns the RCS think carefully for him or herself about whether to purchase this book, knowing full well that not much outside of a few mechanics have changed.
Perhaps also worth mentioning is the change in the artwork. It's never been something I've been especially concerned with, but I've noticed an increasing number of people who have a great appreciation and/or interest in it. The artwork in this book as compared to the RCS is staggeringly different, leaning much more towards heavily detailed and realistic (if, in my opinion, at times cartoonish) illustrations, rather than the "line drawings" seen in previous publications.
I'm actually of two minds regarding my singular complaint. It has been said that too much specialized information appears in this text for it to be truly passable as a player's resource, and for most first-time Ravenloft players this is absolutely true. Here, in this volume, is the complete history of Ravenloft and Count Strahd, the exact effect Ravenloft has on the individual classes and their magic, descriptions of the domains of the entire core and most of the islands and clusters, and many other details that will go a long way towards dispelling a great deal of the setting's mystique. On the other hand, if you plan on running a native campaign, giving your players access to this book may be all but essential. It does no good to be a native of Ravenloft and have no idea of exactly how that is going to shape your character's history and past experiences, to say nothing of the problem with a studious character with a high intelligence score having no idea of the basic geography of the world in which he lives. And of course, at the DM's discretion, this book may also be acceptable reading for longtime Ravenloft players who are already aware of most of the world's ins and outs.
All in all, this is an excellent product that has unfortunately taken a lot of heat from people who feel they wasted their money on a useless reprint and DMs who feel (however justifiably) that it has been titled incorrectly.