I have only been using the free upgrade from version 4.0.X to 4.5 for a short while, but my favorite new feature(s) is (are) tighter web app development integration.
Configuring a web app module for a local tomcat instance is easy, and the new feature of auto launching a browser with the URL for the locally running web app is a good timesaver.
In my opinion, IntelliJ is the best way to develop Java. I have given up running Emacs and JDE. And I am down to using Eclipse and NetBeans to **maybe** 5% of my Java development (probably less).