Why Emacs?

Emacs is still combination editor, operating system and coffee maker of choice for many Java developers.  Unlike garden variety text editors like Vi, Emacs can be extended to be fully functional Java development environment with code completion, refactoring and other high level Java development functions. The Java Development Environment for Emacs provides for all the tools an experienced Java developer needs to be productive. There is no JBoss specific support in Emacs or JDEE, but it's hardly needed.

Why not Emacs?

Emacs users are strange characters that have joined some kind of cult.  You must hit triple key combinations to use Emacs and utter some strange language called Lisp.  Emacs is a lot heavier than Vi and is not always installed.  A frequent criticism of Emacs is that it does "too much" or that "its a great operating system in need of a good editor".  If you need a good editor consider Vi instead.