Months ago I had to use it in a project, but those days gwtquery had a lack of the most useful jquery's methods, mainly due that it was just a prototype exhibited in the Google I/O 2008 .
Then, I decided to add all of the core methods present in jquery and I modified a considerable amount of code including effects, event handling, testing, etc. At the end I produced a patch with about 2500 lines which I sent to Ray, and he included in May 2009.
In the Google I/O 2009, Cromwell gave a conference speaking exclusively about gwtquery and he included me in his credits.
Now I'm planning to contribute this project more frequently.