Is Anyone Using the Google Web Toolkit?
It doesn't have any of the good widgets that exist on most of the google pages that make them great, at least the last time I looked. For instance, there's no pop-up calendar date picker like Google Calendar uses. There's no form field completion popup with scrollbar. So if you want to do anything good, you have to roll your own anyway, or use something like YUI.
If you are going to end up using YUI anyway, it does everything GWT does and more, so you might as well simplify and limit your third party library complexity to one fewer library.
It really doesn't buy you much over JS-only toolkits, IMHO. Unless all you know is Java and you want to make a very very simple website.