I had a year's subscription but did not renew it, being a bit disappointed with the product.
1) The designer injects all Java code adding creation/property setting/event handling to the module you are building in a most unecomic way. Additionally, it is pretty difficult to modularize the application. Compared to Flex, for instance, the number of lines of code gets easily double or triple in a module and seeing the forest from the trees is not easy.
2) Web applications I built only a a couple of years ago CEASED WORKING possibly due to changes in Javascript engines. That was something I did not expect and is very possibly GWT's fault.
3) The GUI designer needs A LOT of CPU power and so does the build phase.
The concept is nice, however, I admit and GWT designer is the only of its kind. The feeling of difficulty in modularizing may well be my lazyness.
EDIT: Shortly after I wrote the above comment Instantiations/Google introduced the UiBinder concept which makes a lot more sense than the original amalgamation. Now it is a lot more professional.