All in all this is a good book. It's not a great book, but it's a solid additional Monster Resource.
I'll hit you with the bad, first. There are three section devoted to already printed monsters with class levels: vampires, kuo-tao and hobgoblins. Now I can understand how this could be useful to some people, but it seems to me it should either be a web supplement or a separate book. Each of these sections takes up more room than a regular monster entry. That means you miss out on several new monsters for each one of these entries. Some of these spots include classes from some the other popular supplements, like the complete books. For me this is a positive because it makes them seem more unique, so I might use them. If you don't own the books the classes are from, they include enough information to still make them usable.
The other bad, was the two maps that were included. Each takes up about a page and were published separately on their website. Which means they simply take up room.
Now the good. Lot's of new monsters, most of which could easily appear on Material Plane. The range of difficulty is vast. They are fun ideas, that would probably not be apparent to players running into them, and seem very easy to run. Several have stats on how to make them familiars or mounts.
Two of the biggest fears were of the inclusion of Mind Flayers who follow Thoon and the Dragon Game. I actually have found these to be the two biggest positives in the book. The Thoon Mind Flayers are not, for the most part, Mind Flayers with Character levels. They're different types of Mind Flayers and Mind Flayer creations. They're all really fun, with varying CR. They're written as a separate sect, but they really don't have to be. There's even an Elder Brain with a CR of 15. This way you can have an elder brain battle before getting to Epics.
The Dragon Game, takes up a little too much room for my liking. There's a lot of fluff here. Dig through and you'll find a real gem. The template that can be applied takes away sorcerer levels form the dragons and replaces them with special abilities that are simple to use and make them seem more like dragons. It also gives you easy ways to make dragons a more intense and plot oriented part of your campaign world.