I agree that the 500mm with various converters is the way to go. When I was on the Canon side I owned 400mm, 500mm, 600mm, and 800mm at various times.
Best all arounder for long lens is 500mm F4, especially if any airline travel is involved. When I switched back to Nikon this spring I wound up selling my Canon stuff and just bought the 500mm VR. I may eventually end up buying a 600mm again just for doing BIF like fishing eagles, but I won't travel with it. I would not buy the 400mm or 800mm again.