Every OpenMQ customer I've met is a happy one. This JMS implementation is now several years old and enjoys a fair number of business critical deployments. Clearly, while the product has been fully open sourced (including HA), it doesn't yet enjoy a large and vibrant user community.
Wotif.com's Greg Luck calls OpenMQ Sun's best kept secret and insisted on presenting OpenMQ at the latest CommunityOne conference. This was a replay of Dave Whitla's presentation in Sydney, back in March. The corresponding slides are here. Greg's talk was recorded and is available on ustream.tv's GlassFish Channel. The video quality isn't great, but the sound is good: part 1, part 2.-
OpenMQ connectivity at this point is Java or C only which could limit MQ's use in some environments. The performance, stability, availability, and support level on the other hand is why people chose and stick to OpenMQ. As covered in the first part of the Wotif.com case study, while message oriented architectures can be key to a modular development and scalable production they are still often not used.