At this point the HD6XXs are the best headphones I own with all the others I have being gaming headsets that range between $40 and $250, so I am relatively new to audiophile stuff.
The clarity is _amazing_, these things are very nice. The earcups are a little bit large in comparison to what I am used to, but they are comfortable. The initial side pressure is just the wrong side of comfortable for me over extended listening sessions (2+ hours), but I also believe that they will loosen up a bit over time and that problem will go away.
The highs can be a little bit more than I enjoy, but that isn't horrible since I don't normally listen to a lot of music with those highs anyway. Open-backed headphones are also amazing (well, at least these are) in how much you can hear outside of the audio experience when the volume is low or music paused. That is pretty cool since with my gaming headsets I normally have to remove one or both earcups to be able to talk with people offline.
My biggest complaint with them so far is that they seem to have a relatively small sound stage and sometimes the imaging (sense of positioning of sound) can get muddled about. To me the sound stage normally feels like it ends somewhere between my shoulders and the outside of the headphones, so it is a very narrow space. This wasn't very surprising as that seems to be a common complaint with the 6XXs/ 650s, but I didn't really understand it until I got to experience it for myself. As a result, I still find gaming to be better with my Turtlebeachs (PX24).
Another common complaint that I saw in reviews and things was that the bass wasn't very "there" with the HD6XXs, but I can't say that I really see it. The bass seems to be good; not overpowering of the mids, but still very present.
Overall I am very pleased with the purchase so far, and am looking forward to when my Schiit stack arrives (to give me a better AMP/DAC than I currently have access to).