After having the HD 6XX headphones for a few weeks:
The sound quality is superb. These are the best headphones I have owned that way (and I go back to the 1970's and Koss Pro4AA's....). The overall sound balance is neutral and relatively uncolored. If you want "character" headphones, these are not it. If you want to hear music and speech really accurately and clearly, it would be hard to do better. The bass goes very deep without sounding unnatural.
Wearing comfort is very good, as the earpads fit around the ears, not on them, so pinna discomfort is not an issue. The open backs allow air circulation. The headband is a bit clampy, but the pressure on the head is relieved by the earpads.
Minor nit-picks: "Midnight Blue"? They just look black to me. Fortunately, I don't really care what they look like, I just want great sound. The left and right marks on the headphones and cables are hard to see without looking closely, making it easy to put the phones on backwards if you just grab for them. I put a dab of white paint on the right cable end, which solved that problem. I'm not wild about the 1/8th to 1/4 inch adapter plug, which looks generic and comes off too easily. The shorter cable has pros and cons--it is easier to handle than the 3 meter original cable for the HD-650, but it means you have to stay closer to what you are plugging into. I may get a 3 meter cable from Sennheiser, since it can be easily swapped. Sennheiser's original adapter plug is also better.