Having finished reading the book, I have to say, I like it quite a bit. Just flipping through it, it did look a bit meagre. I think where it really succeeds is usability: It's easy to drop the presented material into your game with a minimum of effort.
I also like the links between the different sections, e.g. locations, npcs, sample encounters, and new monsters.
While there aren't many monsters, the ones that are in the book are quite cool. Many include a neat new twist that I haven't seen anywhere before. I particularly liked the new take on the frost dwarves (oddly named 'eisk jaat' now).
What I like about the example encounters is that they often include hazards and sometimes use templated or reskinned monsters. I think that's a first.
If the forthcoming 'cosmology' books (Plane Above, Underdark, Demonomicon, etc.) are similar, I'll definitely get them.