A lot of people voice their disappointment with this book, because they expect it to be an in-depth, authoritative guide written for professional designers, and it turns out to be something else. Let me tell you a little secret, design people: it's not "the design bible", it's not "the UX bible", it's not anything bible. It's more of a religious pamphlet aimed at laymen who don't normally think about design in their everyday work, to bring them the gospel of good design practices in an extremely condensed form.
Developers love this book, because it's good (duh!) and also because it comes with recommendations from several luminaries in the field, most notably Jeff Atwood, the co-founder of StackOverflow. I'm no exception. It helped shift my focus from making software that does its job well, to making software that helps its users do their jobs well. It explains in very simple terms why you should care about how users experience and interact with the things you make and how to start thinking about making their interactions more satisfying and rewarding. It also walks you through the typical interaction cycle, from the idea of action that user wants to perform, to the interpretation of feedback they receive; it is a tremendous help when you are trying to 'debug' the interactions and figure out the exact reason why users find your design distracting, irritating or counter-intuitive.
There are sections clarifying the terms you might have heard elsewhere but don't know exactly what they mean (A/B testing, root cause analysis, iterative vs. waterfall approach) or how they might help you improve your design. There is a particularly illuminating chapter explaining why fridge controls and stove controls (among many other things) come in so many different and incompatible designs, how companies are trying to solve this problem with standardization and why standards sometimes create more problems than they solve.
What else? It's also short, well-written and entertaining. The jokes are rare, poignant, and usually delivered with a deadpan snark. To give you an example, "The typewriter was a radical innovation that had a dramatic impact upon office and home writing. It helped provide a role for women in offices as typists and secretaries, which led to the redefinition of the job of secretary to be a dead end rather than the first step toward an executive position". Nice, huh?
To summarize: buy this book if you want to know more about design in general and/or become a better designer to complement your other skills. Don't buy this book if you expect a huge how-to manual or a cookbook aimed at experienced designers.