I've used Google Web Toolkit, and it does what it claims: It allows you to write web apps in Java. GWT has solid abstraction layers, an optimizing compiler, and clever support for image spriting. Using GWT is only marginally harder than writing a desktop application in Java.

Of course, that leaves open a larger question: Do you, personally, want to write web apps in Java? If not, then you'll find GWT to be a clever solution to a problem that you didn't want solved.