I'm going to try to keep this fairly short and sweet and not go into minute details about the bass, mids, treble etc.
At $199, this is one of the best values in headphones, IMHO.
I have a number of "higher end" headphones - HD800, LCD-2, LCD-XC, AKG K812,and Sony MDR-Z1R as well as some "mid-range" headphones like the AKG K7XX, Fostex TX-0, E-MU Teak, and Hifiman HE-560. In terms of sound quality, the HD 6XX is more refined and detailed than any of the mid-range headphones I just mentioned, except maybe the HE-560 (which retails for more than double the price of the HD 6XX). The one that comes the closest in sound quality in the same price range is the K7XX, but I find myself gravitating to the HD 6XX.
Compared to the high-end headphones I have, I never feel like there's anything lacking. They don't have quite the level of detail of the HD800 or K812 or the bass of many of the other HE-560 or LCD-XC, but they are extremely well balanced and listenable.
In terms of bass, midrange, and treble, I would say that they are at least 90% as good as any of the other headphones I own, if not better and do a great job of bringing all of those things together in a smooth, but not laid back way that makes them very enjoyable to listen to. There's plenty of detail, but they never sound hard, sibilant, or "bright".
They're also nice in that there are affordable options for balanced cables and earpads.
The area where they beat many of my higher end headphones is comfort. I find them to be some of (if not the most) comfortable headphones I own.
I guess that wasn't all that short, but hopefully someone will find it useful. If you're getting your first pair of "good" headphones and are on a budget, these are where I would start. Even if you have "flagship" headphones, these are a great set of cans if you like to switch things up. I haven't tried everything out there, nor am I a professional reviewer, but I think you'd have a hard time finding a better sounding pair of headphones at double the price of the HD 6XX.