I cannot speak for the Microsoft WWF, but JBoss has some big customers (both commercial and non-commercial) who gave their trust in using jBPM in a large production environment. You can therefor trust JBoss to have a strong commitment to jBPM now and in the future. There are some important opensource projects like Alfresco (ECM) and Pentaho (BI) that also have commitment to useing jBPM.
The GPD is indeed limited compared to big license based BPM vendors or compared to Microsoft. The tool is however functional and since you can use the source (xml) to adapt the process in areas where there is no support in the GPD (yet) you get all the flexibility you need.
Besides this, I personally (I do not work for JBoss nor for jBPM when it was independent) have found that the jBPM process definition gave me all the flexibility I need.