I'm working on a project right now that uses EXT GWT (GXT) not to be confused with GWT EXT. There is a difference, EXT GWT is the one that is actually produced by the company that wrote ExtJS the javascript library. GWT EXT is a GWT wrapper around the ExtJS library. GXT is native GWT.
Anyways, GXT is still somewhat immature and lacks a solid community that I feel GWT EXT has. However, the future is with GXT, as it's native GWT and actually developed by the company that made ExtJS. GWT EXT is somewhat crippled as the license changed on the ExtJS library, thus slowing the development of GWT EXT.
Overall, I think GWT/GXT is a good solution for developing a web application. I actually quite like hosted mode for development, it makes things quick and easy. You also get the benefit of being able to debug your code as well. Unit testings with JUnit is pretty solid as well. I haven't yet seen a great JavaScript unit testing framework that I felt was mature enough for testing an enterprise application.
For more information on GWT EXT:
http://gwt-ext.com/
For more information on EXT GWT (GXT):
http://extjs.com/products/gxt/
