First pair of HD 660s. I noticed that one of my connectors was pretty damn easy to insert/remove. It still takes a bit of effort, but it was leagues easier than the other connector.

I've owned a few Senns before (600, 650), but I've never had such a disparity between the connector "grip" on the same set of cans.

In terms of sound, there is a very much noticeable channel imbalance. The left cup was 10-20% louder than the right cup. The cups themselves produce this horrible clacking. Turning my head? Clack. Moving my jaw? Clack.

Everything else was fine - typical, reliable Sennheiser plastic build.

EDIT: Returned the first pair. Replacement cans came. Tried them, and the first thing I noticed was that the channel imbalance was even MORE intense than the last ones. I have no idea what Sennheiser is doing, but maybe it's the fault of my seller selling open-box/returned items as "new".

If you happen to hear any creaking when you move your jaw with the headphones on, adjust the bands to be one click shorter (repeat if necessary). If the bottom of the cups touch anything that moves, a horrible cacophony of clacking and creaking ensues. This is my experience, at least. If this doesn't work or becomes too uncomfortable, I personally suggest moving the cups "forward". It reduces the creaking significantly for me, though it doesn't get rid of it entirely.

EDIT 2: Got the third pair after returning the second ones. They're just alright. The right cup is slightly louder than the left cup, but not enough to be a problem. Not going to be returning this one, as I simply don't have the patience. I will fix it on my system with EQ software if I get really annoyed, but, otherwise, it's fine.

Good luck out there, folks!

EDIT 3: Before anyone thinks I hate the product, let me make it clear: I don't. I absolutely adore the sound that these cans offer. Very easy to listen to. I could put these on and get lost for hours. My personal favorite of the Senn 6 line so far. The first return would have been the last if I disliked them.