I've been happily using JBoss MQ for about 3 years now, doing an astounding number of transactions on a regular basis. Getting durable messaging to work reliably took a bit of time, but it performed well once we got it up and running (this book was quite helpful)
The main reason I attended was to get a better idea of one of the main features in the new JBoss Messaging product, which has replaced JBoss MQ. Ovidiu had a great presentation about the latest rev (v1.2) which is supposed to hit GA at the end of this month. From the looks of it, v1.2 will be a significant improvement for High Availability (HA). The team has put quite a bit of effort into providing seamless/transparent failover for clients. JBoss MQ has the concept of HA via a heartbeat and a shared relational store, but v1.2 looks to really move the ball forward. Also of interest - the performance is supposed to be markedly improved.
Here's a post detailing the features in 1.2 by Tim Fox
I can't wait to give it a whirl