I've only had a chance to look through this book so far, but I plan to buy. It passed my main test: one of my players thumbed through it in the bookstore and now dreads me getting the book because of what it will no-doubt put his poor character through. The inventiveness in these creatures means that you can do more with monsters than "he runs up and swings at you," which makes the players think first before they swing.
The usefulness of this book may really depend on what sort of game you're running. I, for one, run an undead-heavy campaign (and am looking forward to Libris Mortis for obvious reasons), and this book seems to have some very inventive undead. It also has several interesting fey creatures, something beyond your standard nixie-pixie-grig type... darker types including the Redcaps. It fleshes out a couple types that get overlooked.
Also of note: the Shifter, Changeling, and Warforged of the Eberron setting are statted here. So, if you like the idea of these as playable races but don't want to use a published setting (like me), this is a good way to get legal access to those races in all their glory. As a DM, the thought of a band of changeling assassins is too good to pass up.
For those who have said that the Eberron and FR entries are wasted space... I honestly think they are space-filler to accomodate the one-monster-to-a-page format (which I think is a great change from previous MMs). If the setting-specific entries weren't there, reviewers would gripe about all the blank white space and a slightly overpriced book would suddenly seem far more expensive.
One complaint: at a glance I only saw one template. Personally, I'm a fan of templates, so this hurts the book's value in my eyes, but it's no big deal. Inventive DMs can always come up with templates based on any nifty creature they find here (example: any of the countless Troll variants presented here could just as easily be applied to, say, orcs or goblins with a bit of work).
Overall, a solid book. The $35 bookstore price may be a bit steep for what you get, but it's definitely worth buying online with the big discount.