I'm on the MD-11. Domestically I don't think it makes a big difference except I would rather wear something that is mine not a headset that has been spit on by 600 guys. Internationally I think having a good headset can make a huge difference. There have been many times where I was able to understand controllers with my Bose headset when the other guys had no idea what was being said.
I just upgraded to the Bose A-20. I love them. If you happen to have a Bose X tryouts into trading it in. I just got $425 for mine and Bose also offers a discount if you happen to keep your CFI current.