I might not be 'unprejudiced' but I've head less problems with jBPM then I had with one or two commercial ones. How mature is it? Well, I've seen commercial vendors change there complete bpm engine by buying a competitor 2 times in 5 years... How mature/stable/... is that.
There are some major (fortune 500) companies that use it, but I'm not in a position to disclose that (NDA). There are some (big) open source projects that also use it, so that might be a reference.
We have an application that uses the BPM to control the workflow within a web application that manages the flow of a user's session. It does not use any of the database flow persistence features since the user session has the lifespan as the modeled business process. Our flow is quite large and is pretty heavily used with about 100,000 sessions per day.
Our biggest problems are related to version control, branch management and revision integration between our various development groups.