The rule engine JBoss Drools uses GWT, it's worked out very well for us allowing us to build a fast and complex app. The base set of widgets and theming with GWT still isn't good enough yet, although 1.5 is an improvement, so we used EXTJS with GWT, using the GWTEXT wrapper. However since the EXTJS license change we now need to migrate to something else, probably Dojo but still using a GWT wrapper.
We have a number of blogs with screenshots with our latest work:
http://blog.athico.com/2008/07/drools-50-m1-new-and-noteworthy.html
http://blog.athico.com/2008/04/brms-for-drools-5.html
http://blog.athico.com/2008/07/integrating-rolodex-to-guvnor-for-image.html
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