GWT is used by many products at Google, including Google Wave and Google AdWords. It's open source, completely free, and used by thousands of developers around the world.
The video below does a really nice feature overview but here are some of my bullet points from the video. You can find more detailed info on the new features here.
- Declarative User Interface - gone are the days of programmatically laying out your application in a Swing type manner. Similar to Adobe Flex, the new UiBinder allows you to lay out your user interface in an XML file and then bind that to a Java class that contains applications logic. This is my favorite new feature!
- Speed Tracer - a new tool that helps you analyze the performance of any web page and understand where the various sources of latency are.
- Easier Styling - with the new UiStyle feature you can write CSS styles and bundle them directly in the template. You get the speed benefit of no extra http roundtrip to fetch an external the stylesheet and prevent name conflicts across your application when using widgets.
- Predictable Performance - provide a consistent look and feel with improved layout panels that behave predictably and load quickly.
- Debug in any Browser - no more requirement to use the embedded browser when debugging Java source code. You can now debug in essentially any browser and use development tools for that browser like Firebug in Firefox.
- Faster Load Times - Developer-guided code splitting allows you to chunk your GWT code into multiple fragments for faster startup.
- Improved IDE Support - the new Eclipse plugin provides support for the UiBinder, client bundling, RPC refactoring, wizards to generate boilerplate code and the new development mode.