On a daily basis I mainly use Flex to develop our Rich Internet Applications (RIAs), but the tools for building RIAs with AJAX have become much more stable and cross browser supported. And with big companies behind things like GWT from Google and YUI from Yahoo, it is now perfectly possible to build stable RIA solutions with AJAX. As Google (among others) have proven for years of course.
Coming from a Java background I find Vaadin interesting and have started to build a small application to try the technology. But instead of building a little example application, I want to build a real working application and do it “the right way”, using the familiar technologies: Spring, Hibernate, etc. and using a proper MVC structure to make a proper separation of concerns in the frontend.