We are using GWT ... in particular GWT-Ext which stands on the shoulders of the Ext JS library. It is an amazing environment to develop in ... and the UI is slick.
The google folks have done a great job putting the core of GWT together. For me, it is the most interesting framework in the Java playground for front end development ... the productivity gain compared to traditional java web frameworks is working out really nicely for us.
I take your point about the exit. The numbers are the ball park that the founders would be happy with to exit.
What would an appropriate level of ownership be for that kind of risk?