What is the current status with users of the OpenEngSB? Is it still in an early state, or are there cases of it being used in live projects?

Well, once there is no more work to do a software is dead :-)

As you might see on github.com/openengsb there is a lot of work going on into various projects. And as we get more and more feedback from adopters there are also interface changes. So there are and will be interface changes between major versions (and only between them; none in minor and micros; we believe in semantic versioning).

With that said some words to the usage in live projects. We've a commercial product based on the OpenEngSB named LogiASB. Well, basically this is the OpenEngSB with domains, tools and workflows specific for the creation of water power plants. They wire their tools, data and engineers together controlling and supporting the development process over those multi-discipline tools. We've three instances at various customers running and also working together with various tool creators to provide support for the OpenEngSB/LogiASB. So, the OpenEngSB is in use at various big customers, but to stay honest, it has not (yet) the distribution as other busses like SMX, Mule, ...