I am interested in the possibility that GWT could serve as the basis for my entire presentation layer.
I would be interested to know if anyone has tried this successful
GWT is relatively new. The compiling process tends to get kinda slow as your code base grows. When we worked with it we found many problems with the layout and rendering of more sophisticated widgets, and the emulator acted totally different from real servers. Also, we had trouble with i18n for right-to-left languages...
All in all, GWT has (the usual?) problems of young technologies. However, it does make certain things really easy, like Ajaxifying as you named it.