I am evaluating WF for use in line of business applications on the web, and I would love to hear some recent first-hand accounts of this technology.
My main interest
We've used WF in a large-ish SharePoint application and I can say it's OK. It has lots of power and flexibility. and, as Kevin mentions, once you grok the underlying concepts of workflows, you can do pretty much anything you want with it.
On the other hand, it has some really serious issues, like lack of versioning, which can really hurt your application in the future. We've been forced to deploy up to 3 parallel versions of the same workflow named xxx-v1, xxx-v2 and xxx-v3 to keep older instances running and have new instances use the updated versions. A real pain in the ass. Oh, and there are also some really non-intuitive concepts in there (correlation tokens, wtf??)