Yahoo! Movies has a good article on the relationships between directors and editors. It focuses specifically on three noteworthy movies in 2007--Michael Clayton, The Kite Runner, and The Eastern Promises.
I've never worked with an editor on any of my projects. For my feature, an editor will provide me the critical distance and fresh perspective needed for the long post-production process, but being that the first cut sets the direction for the rest of the process, I don't know if I am able to let another person do that.