Sorry, but your question is impossible to answer.
What's a "large scale project"? What's "how much"?
The product is extensively QA'd, supports different forms of failover, you can buy support subscriptions for 24x7 support for mission critical deployments etc etc.
It really depends on what you're trying to do. This goes for any product and not just JBossESB. If you think your project is "large scale" and there may be scalability issues, then you'd probably be wise to do a proof of concept first, no matter what solution you're talking about.
As Tom says, you really need to look into your requirements and see how good a fit they are. This goes for any software. A proof of concept is a must, especially for any 'large scale' project.
As things stand, we have lots of customers who are using ESB for 'large scale' projects and we use it internally.
for example core banking that jbossESB server for first layer
get and route messages or other channel
and more instance of jbossAS on more server for respose to requests
and other .....