After a very brief intro into GWT and the tools, I introduced the problem: I asked everyone to enhance a sample web app by pairing up. Every pair of developers was given a different enhancement to the system to add. (There were 6 groups enhancing the system concurrently, for the curious.) In coding up the example app, I had done my best to give the initial code base some decent unit tests (both mocking and GWT-style,) so that people could explore the testability of GWT. I had also tried to carve out small enough classes with enough coherent and singular purpose to allow people to explore maintainability of the code as well.
After the dust had settled, the last check-ins had been merged, and the pizza arrived, the verdict was in: people really liked working with GWT. Given maybe 2 hours to code up a non-trivial feature, nearly all of the groups had succeeded. Some had enough time to go on to contemplate better designing of the system and best practices for testing this code base.
Now, every rose has its thorns. GWT was slow to compile and the error messaging is cryptic, but given the speed of development, ease of use of the platform, and all that you get for free, everyone was bullish on further use of GWT. All were looking forward to a chance with working with GWT again.