I have just read Presenting Windows Workflow Fundation Beta Edition (Sams, 2005, ISBN: 0672328488). This book has been written by several key members of the Microsoft team responsible for creating and marketing Windows Workflow Foundation, and covers WWF at an introductory level.
I really encourage .NET Architects to start learning the basics of this technology. From the readings and practices I have coded I believe we are in front of an emerging powerful new framework, that will be certainly usefull not only for typical workflows but for coding aplication logic in a more comprehensive way.
In my honest opinion the most interesting aspects of WWF are the posibility to represent and code application logic in a "flowchart way"; the extensibility of the framework (in terms of activities and plugable services); and the provided core activities (call web services, methods, etc.) and services (persistance, transactions, etc.).
The most important drawbacks I have found are the complexity to understand the concepts behind, and the caveheats related with tread management (workflow is a parallell thread of the aplication).