Nice question!
There is always stuff for debate, Ross Mason (Mule founder and project lead) posted an article about his view on JBI on
theserverside.
My personal view is that ServiceMix and JBI in general would be a good choice in environments where XML is the predominant message format and you want to go for an open standard based ESB. Having this said, ServiceMix 4 is OSGI based where JBI compliancy is not mandated.
Mule would be my choice in environments where
Java is the predominant technology, so you can use POJO messages, and where open standards are much less of an issue.
Best regards,
Tijs Rademakers