I had a recent experience with a designer who was invited to observe a focus group of prospective users of a large commercial real estate she had designed. She declined the invitation to attend with the following statement: "I have no need to know what people think of the design. I designed it with a specific purpose in mind, and I believe I achieved my goal. What could I learn?"
What arrogance! Well, guess what folks? The users testing the site found it confusing, hard to navigate, difficult to search, and therefore not something they'd be likely to use. I guess if her purpose was to drive people to better sites, she succeeded.
I've used techniques similar to the ones described in this book to test sites I've designed. They work! It's not always fun to hear users tell you what you've created is clumsy, confusing or downright stupid, but if you don't listen to your users and design for them, you are doomed! Don't be like the designer I mentioned above, read this book and learn something. Your sites will be better, and your users will thank you for it.