This book is an absolute must-read for all developers. Especially pertinent to web developers but incorporating the principles of usability into your development process of a client or server app would be invaluable. Krug is a usability expert and that's how makes his living. However if you're not prepared to shell out the big bucks fortunately for you he provides tonnes of examples and even tells you how conduct usability testing yourself.
Another aspect to this book is that it's a complete joy to read. This is not the case for most technical books. There is a light-heartedness to it that helps keep you engaged. Of course, the layout is straight-forward and easy on the eyes. Krug recognizes the hypocrisy of many books that preach about being engaging and interesting in a dull prose.
This was a book I had heard a lot about, but never got around to reading it. I wish I had read this sooner. There are about a dozen projects I can think of off-hand where this information would have been a huge help.
Go get it, read it now. If you've already read it, pick it up again, keep it close at hand. Looking at the majority of what I see at work and online, this should be required reading before people can develop anything.