This book continues to build on the 4th edition core mechanics by providing tons more creatures for DM's to choose from when building encounters.
As mentioned by a previous reviewer, there are lots of variations on existing monsters and fewer brand new monsters. I personally think that is the better way to go. D&D has a lot of good core monsters, no need to include all sorts of bizarre, weird, and silly new creatures. What is much more useful is to creature variations of existing monsters at different encounter levels, and this book just does that.
The encounter groups listed at the end of each monster race entry include both Monster Manual 2 and original Monster Manual creatures, a nice touch.
Lots of humanoid races are included such as Duergar, Bullywugs, Myconids, Centaurs, and other classics from previous editions. They also expanded on some of the existing core races from both the Player's Handbook and Player's Handbook 2: Eladrin, Devas, Humans, Half-Elves, Half-Orcs, and Goliaths.
This book focuses almost exclusively on monster mechanics and it delivers exactly that. However I ding it one star for a combination of little things.
1) There is a great table in the back breaking down monsters by level, listing their type (Skirmisher, Brute, etc.) and the page number. Scattered throughout the book are pre-built encounter groups and they vary in level from the monsters they contain. It would have been trivial to gather up all those encounter groups into a table in the back of the book like they did for the individual monsters, but they didn't. (They didn't do this in the original Monster Manual, which I didn't like either.)
2) There is no good mechanism for adjusting monsters up or down in level based on the adventuring party's level. There are only so many monsters in the book so I think it would add more to the playability to have a way of altering existing monsters based on the target encounter level. I am hoping this will come out in the Dungeon Master's Guide 2 but I'll have to wait and see. (Again, the original Monster Manual didn't have this either.)
Overall a great reference for DM's.