I feel really wise myself :) Don't wanna ask me why? So, I've been delaying learning .NET Workflow Foundation so much time. It's been very wisely, as the forth version of the library has been completely rewritten by the Microsoft and it's not compatible with the 3.x one. The designer works in WPF (smoooothly), the architecture is elegant and (the last, but not least) the library in general is 10 to 100 times faster than the previous version.
So, I've started think where this new wonderful WF could be fit in my current project.