The Denon AH-D600 was actually quite good, and one can see that from going over the thread.
It came out overpriced but then settled into a price range that fit the headphone quality/sound.
The D7100 was basically the same sound but way overpriced. The D400 had to much of a bloated bass (correct me if I'm wrong).
I have the D600 and they compare quite well with headphones in the same $300-$400 price range.
Granted, the D600 is not for everyone, and certainly not neutral the way that term is used here at head-fi, but they give plenty of detail in the music, have a very fun sort of sound that works well with contemporary rock/club/electronic music and are very comfortable. I use mine at work, as they also isolate very well.
If you can give them an audition, certainly do, you may find yourself enjoying them for what they are.
I think part of the problem with this line is that people got put off by their looks. Then the sound wasn't quite the same as the original D2000-D7000 series of headphones, even though I do believe the D600 sounds better than the Denon D2000.
Just my .02$