I've been following MEF project for a while (and guice on the other side of the fence).
Managed Extensibility Framework was built for extensibility from the get-going but I think there are two side of the same coin which is extensibility: internal and external.
VS2010 use MEF to have external extensibility for the editor part, which is great.
In my application I'm looking at MEF from internal extensibility (and composibility) standpoint. I want to build my application in a way I can maintain it, test it, understand it to which I use good design practices some of which call to build
little parts that ultimately compose together to do the work. I want to provide single working product to my customers, and I think MEF is still a great framework for me.
Internal and external extensibility are not mutually excluded and I think they encourage each other. If I already have good internal extensibility model I could not use it for external extensibility, with good design off course.
I would like to see reference and examples for both internal and external extensibility design with MEF because as far as I can tell all of the attention was about external extensibility until now.