For me the test of a good business book is whether it gives me some inspiration and ideas for applying in my own work / business. By this measure "Insanely Simple" is a great business book. Although the examples are, well, simple, it had my synapses firing over and over again with ways to apply the messages in the book. Simply the best business book I have read for a long time.
Ken Segall makes a strong argument that one of the keys to Apple's success is a fierce adherence to the custom and practice of Simplicity. To back this up he takes 10 facets of simplicity and uses a story from his history with Steve Jobs and Apple to illustrate each point. He uses stories from his experience with Dell, IBM and others to show what happens when you embrace complexity instead. The book is simple, the stories fascinating but it's enough to provoke a lot of serious thought about how you run your business and whether making it simpler would make it more effective.
For students of Jobs it's also a useful book, one of the first written by a close insider who can explain a little of HOW Jobs was able to both inspire fierce loyalty and demand freakishly high standards. Segall also makes good case for much of Jobs behaviour being reasonable when viewed in context of what he wanted to achieve. In this respect it's a much better book than the relentlessly tabloid approach taken by Walter Issacson in Steve Jobs: The Exclusive Biography