WF is really a nice platform to develop on. We currently use it for custom workflows in SharePoint 2007 and also are integrating it with our custom project management app to manage some complex workflow issues.

It took a while to understand the varied intricacies of the platform but its very well done, highly extensible (if you've got the time to invest in extending it) and quite performant. If you're starting out on learning WF. get Dharma Shukla's book Essential Worflow Foundation. Its a very good book and describes the foundation of WF very well.

Just a word of caution though... dont use it just for the sake of using it. If you have a simple workflow that can be solved with liner code paths using WF is an overkill. It would add more weight to your application. Where WF shines is a living dynamic workflow where rules keep evolving and so does the flow.

Hope that helps.