In relation to the question of when should you skin your application (desktop apps, not web pages, which are almost always customised), it seems a real fact, that sometimes you just have to do it, whether you like it or not.
In these cases, it might help to peek at some beautiful, specially well designed or just functional custom UIs. Do you have some examples of any of these? Do you use them in your daily life? Had you noticed that the UI was customised?
I'm looking for good customised UIs you can learn from.