If you have used one of AddUI apps - such as baseform.org aware app, you know it looks great.
Actually it goes deeper than that: looking good is a consequence of striving to find for a perfect way to interact with a tool to perform a given task. AddUI is the current result of a long way of exploring and perfecting the user interaction with applications made for the web; it has been used in a host of projects and applications and by many users over the last few years. AddUI contains guidelines, rules and places for things to happen in web applications, providing a simple, predictable and powerful look & feel.
Caring about the user interface and continuously perfecting it also impacts on the tools themselves, and on their function. When you are inside a seamless enviroment, any inconsistence shows up; the result is tighter quality control and better software overall.