So the company I did some Windows Workflow Foundation (WF) consulting for last summer
has recently gone public with how they are using WF in their SaaS application.
You can watch a set of videos here – http://msdn2.microsoft.com/en-us/architecture/bb190630.aspx.
I helped them with their basic WF architecture, as well as with a custom Root model
(the part where they talk about a top-level Sequential Model that runs StateMachine
workflows as children activities), as well as their Workflow Designer for their end-users.
I think they are a great example of the power of using WF in your application and
the benefits you get from using WF to build a cusotmizable pluggable system.