Just received my shiny new Bose A20's. Flew with the Bose X for the last 5 years in the 61 and other than a few snags I was generally happy with them.
The one thing that took a bit getting used to was that the side tone was low, so I always found myself talking louder than necessary.
I can confirm that the new A20 is an improvement to the X. The ANR is noticeably better, the side tone is what I would call normal now and the comfort/fit is better as well. The annoying easily moved volume wheels on the X have been addressed as well. One interesting thing is with the X when you stored it the ear cups were spring loaded together, which in the tropics meant that the sweat was trapped. The A20 design prevents the ear cups contacting each other when its in the storage bag, so I think that will help.