I've been using GWT for a long time now (since it was version 1.1), and even though I looked at MyGWT, one of the first third-party widget libraries, way back when, I haven't used a third-party widget library for application in production. Just some prototypes.
So I'll be learning a bit more and playing with it by reading a book called ExtGWT Rich Internet Application Cookbook (see image at left). I'll make sure to let you know how it goes because I'll be writing a review. So stay tuned in the next couple of weeks.