This is the best application server out there. This open source server beats out some of the $10,000 counterparts. After some initial setup as described in this article, developing for it is easy. If you run into a snag, the mail list at jboss.org is more than helpful, and you get answers faster than you would waiting for a technical support agent at likes of IBM, Oracle, BEA, etc. who will probably give you the run-around.
The small footprint is also amazing. This application server is basic and fast. I highly recommend it for production and development.
Java Programmer, Consultant, and Zealot