I thought WWF would be similar to HWS, but I was wrong. After spending some times to use WWF, I realized that it is so cool and I'm convinced that it will be great if the next version of BTS adopts the technology.
I think it provides a good programming model for human-based workflow as well as structured business process, and rule engine. (It should also be interesting to compare it with that of J2EE.)
By the way, I think the WWF APIs heavily use two design patterns named "dependency injection" and "dynamic service locator". Some of you are already familiar with them. Otherwise, I think understanding those patterns can help to use the APIs. For more information about those patterns, see the Fowler's article at http://www.martinfowler.com/articles/injection.html.