The newest Kenko 1.4X TC works very well with a 5D MK III and the 100-400mmL. However, we are still waiting for Canon to deliver the updated firmware for the 5D MK III that extends AF to F/8.
You can be certain that AF performance will drop a lot, so for birds in flight, it might not work well. If there is ever a 100-400mmL MK II, it may be like a 70-300L which doesn't accept TC's.
If I were going to buy one again, I'd consider the Kenko, it costs less and has no physical interference with most lenses, if not all. Its very good, but not equal to the new Canon TC's.
However, some believe that a TC that works on a lens is better than one that must sit in your bag because it won't physically attach.