I just finished reading this today, and I think it was amazing. It is almost certainly the best written, most informed, and most comprehensive treatment of the subject I have ever read. It doesn't get overly technical and ruin the spirit of the process by overwhelming us with science or literary theory. It is really an A+ treatment of the subject. Highly recommend.
My favorite thing about the book is the fact that the authors warn you about mistakes that people make, but they give examples of why it is bad and speak from experience. For example, in “What is Setting Design” Wolfgang Baur explains why focusing on your world's history is an indulgent distraction from the main goals of design. He even has a side-bar explaining why the temptation to write lengthy histories is so common, but he explains specifically how it doesn't actually enhance the final product. Fantasy history, for history's sake, is an indulgence that we inherited from Tolkien, but it never really enhances the living, shared experience for gamers. This really spoke to me.
Another thing I liked was the typology of fantasy types in Baur's “How Real is Your World?” This sort of thing has been done before, but what I liked about this one is the way he demonstrates the advantages of each as well as the design dangers that come associated with them. In previous works, this advice was painfully obvious and usually linked to the tone of the genre, but here he explores specific design issues associated with the content of the theme.
A third thing I liked was the treatment of unconventional religions in Steve Winter's too-short essay on “Why No Monotheism?” and Zeb Cook's “How to Design a Mystery Cult”. These types of religion are rare in fantasy games, often for good reason, but these essays give the best advice I have so far encountered on how to actually make them useful in a fantasy setting. For me, religion in fantasy settings often feels flat and derivative, so I was inspired by the notion that people are at least considering formats beyond the standard henotheism of D&D.
The only thing I disliked were the essays on “How to Write a World Bible” and “Worldbuilding in Licensed Worlds”. The first was too simplistic. It has the kind of advice you'd give to a film director preparing to make his first period film, not the kind of thing you'd give either to a novice GM writing his or her first world or to an experienced GM looking for ways to work more efficiently. The second essay applies only to people who want to be professional authors, which isn't going to resonate with most people. This essay belonged in Kobold's Game Design volume on how to work in the industry instead of here. I would have preferred an essay on how to adapt a Shared World, like Golarion or Dark Sun, for home use, but virtually none of the advice here helps me.
I have read at least four or five works on the subject of worldbuilding over the years. This is the best treatment on the methods of the subject that I have found. While there are more comprehensive manuals on how to design cultures, demographics, climates, and other specialized topics, I found this to be just the right size and utility for my needs as a worldbuilder-gamer and GM. I genuinely expect to get a lot of use out of it.