I appreciate this subject because the most time consuming and weakness point of many j2ee framework (I mean in developing use cases) is in this part and any improvement will help many of us .
I want to know if any successful experiment in GWT (Google Web Toolkit) because the idea writing your front end with Java is delightful But Is it cool changing it's custom component and adding extra functionality ?

G4JSF (JSF wrapper around GWT) worked fine for me.
except for internationalization that time GWT internationalization was poor,
I read some where they have fixed the problem now. but haven't test it.

I used several other JSF Ajax enabled framework and all of them worked fine for me
except in cases that they had to be used in portals.
looks like they are fixing Portlet and Ajax conflicts in JSR 286 (Portlet 2)