To be honest, I've never had an MCollective deployment Just Work(tm). It's always taken some serious debugging to figure out what the heck went wrong this time.

RabbitMQ works but is slightly problematic since the authors have a morbid penchant for not wanting to support anything but Apache ActiveMQ. Ask a question about MCollective and RabbitMQ and the answer you get is 'switch to ActiveMQ'.