Q:

How viable is "Windows Workflow Foundation"? What alternatives should I consider?

I started learning WF 3.0 back a few years ago and never quite got the hang of it.  Has anyone learned WF 4.0 (now that it has been rewritten)?  
Is it a viable platform worth considering? 
What alternatives exist?

A:

At my last job we had a huge plan to use WF4.  We were planning to build a single end-to-end platform to replace the 5+ disparate pieces that existed.  
I will limit my comments to two areas:

It has not been around long enough.  With WF4 being such a drastic change from WF3, the product has not been in the wild long enough to establish a community that can sustain and help itself.  You will encounter issues that will be difficult to resolve, and you will find little help online (and dig through countless unhelpful pages relating to WF3.)
Subjectively, it's just a new fangled hammer... You have to decide if the amount of time you will have to invest just to become comfortable with it is worth spending.  Instead of trying to shoe-horn the application into WF4, I would rather have spent that time building a home-grown workflow system that worked exactly how the application needed.

I admit that the State Machine concept is very appealing to me, but I'm also a proponent of building things from the ground up rather than using (and becoming dependent upon) existing frameworks.  

