I've read a bit about Windows Workflow Foundation (WF) in the past, but I've never done real-world with it. Recently I've been doing a bit of work helping a client get a proof of concept solution up and running for an SOA based forms workflow environment.
I've been using state based workflows, getting my head around the event handling model, the way the workflow interacts with the host (especially interesting when the host is ASP.NET web services - and no, I'm not talking about workflows exposed as web services), developing custom activities for AD interactions and sending emails (via other web services) and all kinds of other interesting things like error handling and so forth.
I must say, my heads still spinning with all the new information I've been cramming into it. WF is just so big & deep & flexible. I always thought it was a bit lightweight and only designed to help people with UI wizard-style forms, but it's oh so much more than that. There's a whole world of possibilities that have opened up before my eyes :-) I think I'll be getting into this a bit more over the coming months as it's very, very interesting stuff.