What makes them so nice? Well, for starters they're very light. They're also very comfortable, with real leather memory-foam earpads.
I only got a chance to wear them for about 5 minutes, but Sennheiser reps said they're designed to be worn for hours and I believe them.
My spot impression as far as their sound goes is that they have a warm, natural sound, that's very detailed. These are not bass-heavy headphones; rather they have a balanced, audiophile sound profile.
I also liked that the plug had a hinge in it, so you can go with a straight plug or a right-angle plug. More headphones should offer this feature.
Other extras include an Apple-friendly inline remote with microphone.
Sennheiser present the Momentum as new-for-CES product, but it's been shipping for a few months. I look forward to giving them a longer listen to see how they stack up to other higher-end headphones back at the office.
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