I've used GWT for about a year, and I like it because I like static typing more so than I like java the language. And yes, i've tried various dynamic languages (ruby and javascript), and i still prefer the statically typed ones for large projects with a large number of developers over a large time frame. I also know that not everyone agrees with that, and if you don't, GWT may not be the best tool for you. However, it works well for me. And if you ignore the language debate, the toolset is very powerful.
(I also hope scala-gwt will become real-world-usable some day :) )