Flexible Workflow Management Systems?

Paul Roberts asks: "I just started on my new job and I have to develop, in J2EE, a workflow management system (WMS) for the financial department. I'm both new to J2EE and WMS, so I could really use any help that experienced Slashdot readers have to offer. I have been warned of frequent changes in the process flows, due to changes in the law or management vision, and the system should be able to handle these, including any retroactive changes that may need to be made. As a bonus, the system could produce personalized letters, but if that feature isn't available, we can program that ourselves. I've been searching the web to find existing solutions, Sourceforge gave a list of 2 pages of active projects (and many more inactive projects), OpenSymphony has OSWorkflow, JBoss now includes jBPM. Of all those WM systems around, what are your experiences, what are the things I should look out for - both in features that should be present and in pitfalls to be avoided?"