They're great, that's undeniable, though honestly I was expecting them to be even better that what they are. I already have the Senn Momentums which were my default headphones for some years, and basically they're almost, almost the same headphone. HD6XX are better, that's undeniable, but not a night and day difference.
They have a wider sound, though not so big actually, enough to don't feel the sound so compressed in your head. and there's some more detail. And bass, how some people can say these are not bassy? Their bass is different from the Momentums, but it's there with a lot of presence.
So to me, it's a matter of diminishing returns I guess, for the full price I maybe could have some regrets (Or not, actually) but for $200, I think they are, probably, the best you can get for the price, and now are my main headphones, I'll leave the Momentums as backup, since these have the same signature (nice mids, bassy and smooth treble) but better.
Build quality seems Ok, it's mainly plastic, but feels solid. At first the clamping force felt weird, but they become comfortable soon enough.
Overall, I don't have any complaints. If you have a Momentum, or maybe another close model like HD598 or HD600, you probably want to consider this purchase, but if you have an M50 or something else with a different signature, I totally recommend this set. It can be a brave new world to your ears.