It seems like there is a confusion around the meaning of Enterprise Service Bus (ESB). To some ESB is a specific product offering from vendors such as IBM, Oracle, BEA etc. To others ESB is a set of technologies such as messaging, application server, web services, data services integrated together -- basically a technology fabric that lets users unlock the value of SOA. I tend to a agree with the later definition.
ESB is a specific implementation of SOA and would have to be combination of various technologies integrated using SOA paradigms and standards.
Once we have the definition right, we can start asking ourselves questions: "What does it mean to manage an ESB?" In my view, based on the later definition of the term ESB, it means that all components of ESB must be managed in a single inform fashion -- not just an individual product offering. "How do we measure the quality of service provided by ESB?" This is a much more complex question to answer and not just due to the sheer complexity of technologies involved but also due to the different perceptions/definitions of what QOS actually means for different users and stakeholders of the SOA/ESB environment.
At any rate, proper definition of ESB is essential before more complex questions can be asked and answered.