I've written several packages for Emacs. The one I'm currently making available is DmsSig , my automatic signature management package (and "personality management" package). (Yes, everyone writes one of these. This one is mine. I think it's pretty cool.)
I haven't hacked on it in a long time, but I use it constantly.
Emacs is the One True Editor (tm). Thou shalt have no other editors before it!
Is (Eclipse, JBuilder, IntelliJ, whatever) better? Perhaps it has tighter integration with it's particilar environment.
Emacs does everything from editing Java (and C, and C++, and HTML, and about everything else) to reading mail and editing this Wiki-like structure (and rendering it into HTML). I find that useful. You are welcome to use whatever you like.