Received my replacement pair and am happy again. Issues I'd had was that the left side developed a scratchy, static-ey noise with any movement (walking, vibrations in the bus, chewing, any movement). It started off as just a little, but over time worsened until noise cancelling was entirely unusable. Also had the effect of making transparency mode sound severely over-amplified, also unusable. In either mode it would make a squealing feedback sound if I was talking at all. By the end it was just starting to show up even in Off mode (which I think may still be using NC but just barely, but I have no idea really.) My wife's pair were doing the same thing but hadn't degraded to the same degree mine had. We bought both pairs on release day, and based on serials I suspect they came from the same manufacturing batch, so I am hoping it was just a fluke. The timing of the issues we were having coincided with the 2C54 firmware, so I held out for a bit thinking it may have been the cause. Since they haven't pushed out another since then, for all I know it may have been a factor but I suspect a hardware fault with the microphone that is used for NC or some part of that system.
Now that I have a replacement pair I realize that the sound quality may have also been compromised, as these seem to sound better as well. No scratchy or squealing sounds, NC seems good enough although I agree with some that it feels lighter than when they first came out, transparency mode sounds close to normal hearing and not over-amplified. I really hope these ones last.