As I mentioned my post about Guice, I'm finally motivated to start playing with code again.
One issue I found out of the gate was the CSS support. I found a post documenting CSS in GWT that is helpful. However, it appears that all instances of Button use the same class (gwt-Button). After a little digging I found the addStyleName and setStyleName methods. This allows you to assign a CSS class name to any object to allow it to be styled in CSS. This alleviates some of the fears I had about having too much of the look/feel in your code.
The biggest conclusion I've drawn, and probably the most obvious, is that GWT appears to be really good for building web applications with a very small amount of pages. If you look at gmail or Google Reader, these are obvious model applications. There is heavy use of AJAX, keyboard interaction, and other rich navigation. Gmail/Google Reader are heavily data driven, but the data is all the same type (email, blog posts). GWT is certainly something I will keep in mind for future projects, but it is a very specific 'hammer' for a very specific use.