I use the 2X on both the 70-200 2.8 II and the 300 2.8 IS for stationary subjects when there is no other way to get the shot. As has been mentioned by others; to get the best from either combination requires good technique, stop down a bit, tripod, remote release, good post work flow.
Otherwise, my opinion is that the results do not meet my image quality standards. And BIF with 2x, forget about it.
Here's an example shot with the 7D, 300 2.8 IS, 2X TC MK II.