We're having to add 'enterprisey' integration to our application so I've gone for MEF as a plug-in container.  We're both retrieving external data and running commands into external systems (for example, we retrieving costs from a system and also raising purchase order requests).

I've gone for a Directory catalogue (with a file-watcher so we can have live updates when new DLLs are dropped into place).

Plug-In interfaces and DTO definitions are defined in their own project for the plug-ins to reference, so I feel we have pretty good separation from the main application.

Anyone got any war-stories regarding MEF? Anything going to catch me out?

Note: we're .NET 3.5, so I got MEF from Codeplex and recompiled with a strong name.