I used it for about 8 months, Hated it. Its like a big wall between you and the DOM.
I guess its good of you hate html and javascript. But once you grow up and realise your making a web page and need to control the DOM in a particular way, it just gets in your way.
Though it's worth noting that Inbox and similar properties do not use it for UI code, but only (exclusively) for business logic below the UI layer. UI code is written in pure JavaScript/Closure.
Moreover Newsdesk, a reasonably large media monitoring platform is pretty much solely based on GWT, though it looks like they are moving away from it in their new development.
I guess its good of you hate html and javascript. But once you grow up and realise your making a web page and need to control the DOM in a particular way, it just gets in your way.