Btw, I'm using RabbitMQ, and I love it. My needs do not include high load or high availability though so I can't speak for that. So what's nice about it? AMQP (you automatically get lots of tools, docs, "expectations", etc), very friendly and active mailing lists, small & clean code base, small footprint, simple, active (more features are always being added).
What I don't like about RabbitMQ? While it's FLOSS inside out, its development isn't exactly a "community" work. For example, I can't report an issue, attach a patch, and receive a reviewer/committer feedback about it and possibly get it in. In fact, I can't even report an issue into their issue tracking system -- I just have the mailing list. That said, I believe they said they are going to fix that part "soon".