Since I've been using a par of ATHM50X as my main monitoring cans for about a year, I'll chime in.
Firstly,I should say that I'm not a great lover of headphones, if there's any alternative. I dislike the feel of them on my head & ears, I dislike the fact that they shut out the sonic ambiance & I dislike the "in your head" presentation of most recorded material (binaural excepted)!
But when sealed headphones are needed to do a job, I find the M50Xs to be a pretty good all round choice. They have good isolation, they're lightweight & fairly comfortable - they fit firmly, but without pressing the frames of my eyeglasses against my largish ears. They fold up into a kind of ball (a bit like the Sony MDR7506) & are fairly tough, with a choice of chunky cables and & locking connector. Mine are constantly in and out of my bag & have been dropped quite a few times without problems. The money seems to have been spent on making them functional & there's no bling. Nor are there any loops of delicate cable to get caught on things.
Of course, if you're using them at a desk, most of this portability talk won't matter much. Sound-wise they're quite pleasant to listen through. They're fairly open sounding, yet don't have a "tipped up" frequency response, although there's some peaking at the top end @ 10kHz or so. The midrange can sound a bit recessed, but it's ok. The biggest gripe which I have with them is the unpleasant hump in the 100-200Hz bass region. It's not that kind of boxy resonance which a lot of closed cans exhibit, but a wider hump which just sounds as if someone's nudged a fader on a graphic eq. (Actually, you could probably eq this out, but that's another conversation...)
It's a while since I've heard DT770s, but I remember them as overly bright, & a bit scooped in the midrange. What else is there which hasn't been mentioned? I used to use DT250s. Still do sometimes. Flatter midrange, but much more enclosed sounding. The oval shaped pads pinch my ears & they don't fold up. MDR7506s might be as good in the midrange, but they're painful at the top end. HD25s, beloved of film & TV PSMs, are great in the midrange dialog region, quite dynamic, but excruciatingly painful (physically) on the ears. A set of the HD25 drivers in an M50 shell would be just perfect!
I'm not sure if there's such a thing as a truly accurate closed back headphone. Is isolation that important to you when mixing? HD600s would be my choice if I had to use cans for critical eq or balance adjustments. But they're open back & more money!
I think it's a case of ask 10 people, and get 20 different answers. But maybe there's a consensus emerging...