Not too long ago I wrote about Drupal's usability history and its future
In that post I wrote:
Visual design and visual impact time and time again translate directly into a perception of usability from the user perspective. For some reason (and you will have to consult with psychologists for why) people gravitate towards the pretty and the attractive. Make something look good and people tend to rate their experience with that thing higher.
And just the other day A List Apart wrote an amazing piece on just this topic. The article is called "In Defense of Eye Candy". It is an absolute must read for anyone who ponders user experience and wants to know more about why its so important for an application to 'look good'.
By making your application look good, you may actually make it work better. That's not hyperbole. The article cites two studies (one found here) that come to this conclusion.