A few thoughts:
1. Very difficult topic to write about. There's so much uncertainty involved that it's almost impossible to even agree on the basic assumptions of the book.
2. The writing is incredibly thorough, given the assumptions, but also hard to understand. You need to follow the arguments closely and reread sections to fully understand their implications.

Overall, interesting and thought-provoking book even though the basic assumptions are debatable

P.S. (6 months later) Looking back on this book I think a major theme is encapsulated by the story of the AI Alice, the paperclip maximizer. In this story, Alice is charged with collecting as many paperclips as she can. She goes to achieves this goal by transforming the entire universe into a paperclip factory, and in the process destroying all life in the universe. (For the full story see wiki.lesswrong.com/wiki/Paperclip_maximizer) Now the main lesson is that what we consider human values won't spontaneously arise in machines. And as shown in the story of Alice this could be dangerous for humans. Nick visits this theme again and again throughout his book. We need to be very careful and teach machines human values and not assume that these values will arise automatically.