Well, I've only listened to them for a few hours, but, unless things change.....
WOW. These are PHENOMENAL!!! The HD 280 PRO has better noise isolation and "collapsibility," but, other than those two items, these are better in every way. The first thing I should mention: Don't be hesitant about the plastic materials. This is a different, better, stronger, and thicker plastic; I don't think Sennheiser used plastic to be cheap -- at least, that's not the primary reason -- but rather to reduce weight. The headband is much more comfortable than that of the 280, even though this one seems a tad thinner. The clamping force is a little tight for me, but I know that will loosen over time, and it's not unbearable. Once I determined the proper height setting, they were very secure; head-jerking didn't move them around much. The ear pads themselves are supremely comfortable.
The sound is, to quote Jim Morrison, "pure, unbounded joy." Someone said the 280 has better bass; I respectfully disagree. All sonic frequencies seem better here. I don't have much experience with headphones, but I can confidently say these are the best-sounding headphones I've ever heard.
Sorry, I don't wish to turn this into a full review! Let's just say, once again, WOW.