It looks like Don Norman, Mr. Design of Everyday Things, has another major hit book in his hand: Emotional Design.  I ordered my copy after reading first few paragraphs of the first chapter titled Attractive Things Work Better (245K PDF).

The core idea is this: if it looks good, you can use it more effectively with less confusion than if it didn't look good.  When you think about it, it's obvious.  Even if you are a talented musician, your performance will suffer if you hated playing music.  Of course, it can't be stretched too far.  I remember reading somewhere that, when a woman is too beautiful, one can have, er, physical difficulties.