<p>We need to add WorkFlow to our Spring managed application. Does anyone have any useful experience in using any of the myriad of OSS Work Flow solutions? Which one is best? Which one integrates with Spring best? Which ones should we avoid?</p>

If you only need some simple process orchestration, Spring's own
Web Flow
, despite its name can serve as a orchestration task manager.   If you need to preserve state for several days then you will need to become an 'early adopter' of one of the open-source projects.  You may want to look at
Eclipse's BPEL project
.
My hunch is that once a clearer picture of the BPEL/BPM/Workflow space emerges you will see Spring provide an abstraction layer the same way they have for JDBC, Transactions, ORM frameworks etc...