I was chatting with my friend, a superb iOS developer, and we talked about how people generally cannot appreciate good UI. Also it seems UI in mobile is a lot worse than UI in web.
Although you cannot say UI is art, but like programming, it is certainly a craftsmanship. There is a lot of decent programmers but I would say not all of them are decent UI programmer, because it not only requires programming craftsmanship but also UI craftsmanship.
I am not a good writer, so I will just collect some good and bad examples: