People are less rational than they think they are, in predictable ways. Kahneman’s classic book offers a wide variety of interesting psychological biases, thoughtfully ordered. The writing is continually engaging, and there are enough examples to ensure that his point gets across. He also intentionally makes everything useful to the reader, concluding each chapter with examples of how one might apply the ideas in day-to-day life.
I've often been very cynical regarding psychology, but it is hard to dispute the results of the experiments and the large amount of corroborating evidence for the various biases. The existence of phenomena like risk aversion, gambling to avoid losses, and WYSIATI (what you see is all there is) seems to be beyond reasonable doubt, and it’s both humorous and scary to read a catalogued list of deficiencies in our mental models. That being said, I am still not entirely confident in the explanations for all of these facts. That's where psychology typically becomes pseudosciency and accordingly where I am least comfortable. But regardless of the qualms I have with psychology as a field, this book deserves its reputation as a must-read. To understand yourself, you must know the limits of your rationality.
EDIT (Dec 2021): I read this book a LONG time ago, and it is severely due for a re-read. Consequently, the highlights below are a small (and poorly chosen) sample of the valuable insights in the book – many of which have stuck with me.