We built a massive state machine based workflow in WF3. It has limped along until .Net 4.0, but we've hit a brick wall now as they have killed the following assemblies in .Net 4.5 (all legacy WF3/3.5 assemblies basically):

    System.Workflow.Activities.dll
    System.Workflow.ComponentModel.dll
    System.Workflow.Runtime.dll
    System.WorkflowServices.dll
    Microsoft.Workflow.DebugController.dll
    Microsoft.Workflow.Compiler.exe
    Wfc.exe

Our 4.5 upgrade prospects are going to be problematic I think as we've got to port everything to the WF4 process model and runtime before we can drag everything up.

I've heard a lot of poeple say "just leave it as 4.0", but we've been there since day one with .Net 1.0 and I'll tell you it's not fun being on the support trailing edge when you have to do a massive port.