Been using GWT for almost a decade. It makes it easy to develop and maintain enormous and complex web applications, which would otherwise be difficult if you had to rely only on JavaScript (both the language and the toolchain).<p>Being able to write the entire application, from the client and the domain model on down to the backend, in the same language, re-using classes across the entire application,
is a big advantage.<p>In addition, being a very mature toolkit by now, GWT has A LOT of features to make life easier. Transparent RPC/serialization, code obfuscation/deobfuscation, IDE integrated debugging, i18n, resource bundling, code splitting, JavaScript interoperability, and a lot of other stuff.<p>Current project: Real-time multiplayer GTA-style game based on HTML5 canvas:<p><a href="http://www.webworks.dk/html5engine" rel="nofollow">http://www.webworks.dk/html5engine</a>