I've been using this famous pair for 3 months. And I gotta say, these are the type of headphones that grow on you when you use them long enough, even if they don't beat the open backs in performance. But as for closed backs, these are still some of the best on the market.
My first impressions for DT770 were similar to other people. I was impressed by their technical performance. They've got soundstage and imaging that's almost on par with some intimate open backs, but obviously not as open sounding. It's easy to hear different sounds and their directionalities.
The sound signature is bright and detailed, with a slight v-shape tuning that doesn't make the mids hard to hear. The bass response is what I would call perfect in Harman terms. It's got presence and doesn't bleed into mids. The mid response is flat as it should be for studio headphones. The treble is definitely bright and sharp at stock, but I found that it does get softer with enough wear. It's easy to EQ the treble region. I went with the the DIY paper layer mod. It gets rid of sibilance and harshness for good. But even without this mod, treble is not so rough that it's a problem with modest listening volume.
I'll be keeping my D770. They're the only wired closed backs that I need. The sound of them is so good, and even if the treble bothers you, it's easy to reduce it with either EQ or DIY mod. And yes, I recommend these headphones over M50X and K371.