Received my HD6XX yesterday. Had a couple of hours listening, here are some fist impressions for those who might consider joining the drop but are still in doubt. As a background: I own several headphones that I know really well: Shure SRH240, Grado SR80e, Sennheiser PX100II, HD518, HD598, AKG K240, Beyerdynamic DT770 250ohm, Audio Technica ATH-M50x, Sony MDR 7506, Koss Porta Pro, Koss KPH 30i. I listened through both a Cambridge Audio DacMagic XS and a Schiit Magni 2 amp (connected to a Denon DCD 700 CD player). I also own Sonarworks headphone calibration software, which I didn't use exensively with the HD6XX yet (just as a check, my first impression is, it doesn't really need any correction).
In one sentence: the HD6XX is easily better than any of above mentioned headphones. Yes, the DT770 has deeper bass and works great with electronic music and the SR80e might work better with some jazz recordings. But the best point of the HD6XX is that they sound excellent with ALL genres of music. They do not do anything wrong in my opinion. They are a comfortable fit. A bit snug, but personally, I can wear them for hours on end. They combine an almost flat frequency response with an exciting, lush, almost euphonic sound. I read a lot of comments of the HD650/6XX sounding "veiled". Not to my ears, I think absence of distortion/grainyness/boosted highs is being mistaken for lack of transparency. They do have a warm tilt to the sound, but sound a lot more refined, detailed and transparent than any of the other headphones I own. If I have to name one thing I don't like about these headphones, it's the fact that I haven't bought them earlier. They are very special. They make you listen to your music, not to your gear.
Update (31-12-2017): I listened to a lot of music since I got my HD6XX more than a week ago, so I thought I'd share what I've learnt. Of course, as I said before, the HD6XX makes you listen to and enjoy your music first and foremost. But I have to admit they do make you aware of some differences if you switch your gear from one setup to another. So if you're perfectly happy with your current setup, stop reading ;)
For instance, with my own gear, the combination laptop + DacMagic XS seemed to work better with the HD6XX than my other setup, which is a CD player (and a Logitech Squeezebox) connected to an integrated amplifier, and a Magni 2 amp, connected to tape out. The CD player sounded somewhat congested, with less vocal clarity compared to the laptop + DacMagic XS. The Squeezebox sat somewhere in between. No dramatic differences, but the HD6XX let's you hear them quite easily nonetheless.
Maybe the CD player would sound better if connected directly to the Magni, but I didn't want to meddle with my cables. Because my CD player and Squeezebox both have digital out, I decided to buy a DacMagic 100, and put it between both sources and the Magni, to see if that would improve things. It turned out to be a good move: voices sound a lot clearer now, there's more fine detail and there's a bit more smoothness to the sound, with a slightly wider / more "relaxed" soundstage (if there's such a thing). The DacMagic 100 + Magni now has the edge over the combination of laptop + Dacmagic XS, imo.
I thought I'd share this story, not to say you should buy this or that dac or amp (there's a lot more good stuff besides Cambridge Audio and Schiit), but to show how the HD6XX will let you notice little differences in your setup. You can enjoy them straight from the "headphone out" of your CD player, mixer or other "high power" source (they're not sensitive enough to play from a smartphone) and they'll sound great, but they will scale really well when you upgrade your equipment. Now back to the music, after all, that's what it's all about.
All the best and a happy 2018!