We've been using JBoss for about 5 years now and we've always found it to be rock solid. We've had experience with JBoss in various environments and also had development experience with other application servers such as Haht, SilverStream, BEA WebLogic and WebSphere. In terms of resource efficiency, ease of deployment and support of rapid application development I rate JBoss at the top of the list.
At Sony Singapore, we used JBoss to drive Citibank credit card transactions because the Haht server could not interface with the JNI adapter from Citibank - running on Sun Solaris.
At the Department of Education and Training, NSW we are running our Response Tester application with JBoss on a True64 platform with Java 1.4.1 - without a hiccup on the container.
We developed our FlexCorp framework around JBoss because we had access to MBeans infrastructure well before other application servers and this was an important feature for our development.
We chose JBoss because it is a solid, scalable EJB container. It is fast and we can run it with less resources than other application servers. Our philosophy is about simple and reliable IT solutions and JBoss is certainly compatible with our aims. For us the JBoss choice was not a gamble, it was just sensible business practice.