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Being able to work in Java in a manner resembling Swing is nice, but it also reminds me how long it takes to create even a simple UI. Learning how to use all of the GWT-specific classes (and learning which standard Java classes and libraries to not use) would take time similar to that of learning how to use components in an XML syntax. Not being able to access REST or SOAP interfaces directly without being on the same host/port is a major drawback.