After spending a couple of weeks with the Momentums, I'm returning them. These cans sound too rich for my liking, which could be a result of how they sit on my ears (feels like my ears are stuffed in there a little tight, but they do fit around them). My ears are average size (55cm if memory serves). I played around with various headband positions (slightly tilted forward, slightly tilted back) and clamping pressures, and found only minor variations in frequency response. More clamp = better isolation and bass, but more (perceived) rolloff in the treble.
Compared to my open can setup (HD650-->Zu Mobius MKII-->Dacmini PX), they sounded a tad congested and forward in presentation. Still highly enjoyable and musical mind you. For fun, I ran the Momentums through the Dacmini and also the ALO Audio Rx MKII. Didn't hear any variance in frequency response compared driving with an iPhone or iPad (which Tyll suspected in his review).
Compared to my closed can setup (iPad-->Logitech UE9000), Momentums sounded more neutral (UE's are a tad dark in active circuit mode) but still forward in presentation. I prefer my UE's for around the house listening because they are wireless noise cancelers. Less hassle for chores or for vacating the recliner to grab a beer. And the active circuit does a decent job of removing fridge noise from the nearby kitchen. Momentum bass is ever-so-slightly emphasized, while the UE9000's active circuit mode is unabashedly bass faced. But I like being bass faced around the house. :-) Compared to UE9000 in passive mode, Momentum had better mids but less sparkle.
Like others in this thread, I felt the right pad was not always sealing as well as the left. But noise isolation is good.
Overall these were highly enjoyable. But my ears fit a little too snugly inside the pads. I'm thinking that caused them to sound a little too meaty and rich. Regardless, I'm excited that sealed cans are making so much progress.