I have used GWT/GXT with spring for a huge project during the last 4 months and everything works great. We have increased our development speed with a factor of 5 by migrating to GWT-RPC. The reason is that we had a lot of old code that needed to be refactored. By moving ALL the code to java we can easily use eclipse to refactor the project without fear of breaking the communication between the server and the RIA app.
I can recommend this approach. The only thing we have found that might become a problem is the size if the generated files. At the moment our app is about 11Mb of javascript code. This makes it a little bit slow to start (especially on a slow connection) and since this is only a javascript page you get no progress for the download. GWT is looking at breaking down the code into smaller chunks for a faster start so I guess it is only a matter of time before this problem is solved but it is not solved yet....