I actually traded my HDJ-2000 for a brand-new M-80 with Val, and I must say that I got what I was hoping for. As much as I loved the HDJ-2000s they weren't as portable for me and I was hoping the M-80s could be like a portable version of those, at least when it comes to having similar bass, warmth, soundstage and detail retrieval. I'm not disappointed at all. My only minor complaint about the M-80s is isolation. Now granted, I take NYC subways so overall the M-80s isolates well when not in a train, but not as much to diminish engine rumble (a la HD25).
Another thing that makes them very different from a company like Monster - well, if you get the M-80s, open the box and feel the construction of these cans... then go to your nearest Best Buy check out the $200-300 plastic by Monster. With the M-80s, you get a nice balance when it comes to style, sound, and construction.