I've worked on 2 large GWT apps, neither backed by Java (one was backed by C++, the other was backed by Perl). Using it to create web applications is really nice, but it's a bit bulky when it comes to some UI work (though, UIBinder helps a lot when you just want to write HTML).
Java is an overall great language. While it does have it flaws, it helps protect you from lots of stupid programming mistakes and it has pretty good performance.
GWT is definitely heavy, considering all its functionality from minifying js, js splitting to optimize page load, multiple js output for browser specific optimization and to reduce js size, generating sprites, merging css, images, etc.
The best part of GWT is the testability aspect via MVP pattern if you compare it against weiting in pure JS which is usually hard to write headless test automation.
Java is an overall great language. While it does have it flaws, it helps protect you from lots of stupid programming mistakes and it has pretty good performance.