I have used GTK+ in the past for a multi-platform application. I found it relatively simple to learn and use. To my mind the main advantage of GTK+ is that you will be able to port your application to other windowing systems. And the main disadvantage is that it will not look exactly like other windows applications. If you are doing cross platform work or are already very familiar with GTK+ (and don't have time to spend learning a new toolkit), I would recommend it.
