It depends on what you want to do (as for most tools anyway).
However, if you want to make a "not-so-fancy", but stable GUI for your application that "should", and in most cases really do the same, and look the same in various browsers without tricks, GWT is a good choice, the best I know about. Explanation: Google is definitely motivated to keep it in sync with the most browsers and the newest technologies, and definitely has enough resources to do it. Yes, you adhere to another giant instead of doing your own stuff. Question: what is your job? To provide the same service to the widest rage of users through a web interface with the smallest effort - or make a shiny, exceptional web portal which has the coolest features on the newest platforms.
I picked GWT because of my answers to the above questions - and it does what I wanted it to do: in about 2 weeks after installation I had an acceptable web interface for an internal server monitoring system - although I had experience with Swing. (No, I have not used the default look and feel and yes, I used CSS and classes for presenting logical information :-) )
Check your current and planned tasks - and choose the right tool for them.