I recently spent quite some time in the System.AddIn namespace. I really like the new extensibility model and once you take the initial hurdle of understanding the infrastructure (or pipepline in MAF-speak) it is a really sensible model.
To get you started more easily the CLR AddIn guys (Jesse, Jack, TQ) have published a tool that allows code-gen’ing the complete pipeline from the contract. Give it a try!
leastprivilege.com 