Pros: Comfort, solid build, not fatiguing, fun!
Cons: MSRP $$$, Earpads can get heated, weighty - not suitable for portable use as Denon intended
Bought these used off the FS forums. I had been interested in these since release, but due to negative reviews did not think it was worth pulling the trigger when they sold for $500. At the price I purchased them at, I am quite happy with them. So long as you know what you're getting, I believe the D600 is a headphone that pleases.
The lower end has a very significant hump that can sound exceptional with certain recordings. I've heard details in the bass on certain songs that I never heard before because it is so audible on the D600s compared to the rest of the frequency range. It also makes you want to groove when you listen to dance-ish songs. You can definitely put these on your to-buy list if you enjoy a good time with a colored sound and a bumped bass. Do not expect strings to sound sweet and rich - midrange/upper mids seem to be somewhat lacking on these headphones. They sound significantly further in the background on recordings than more neutral headphones like the Paradox present violins/violas/etc. Female vocals sound natural but slightly recessed.
They may not be power hungry, but respond very favorably to amplification. I find that driving them out of the Cowon J3 alone delivers a somewhat lacking sound. Adding a PA2V2 seemed to help tremendously. Classical, orchestral and a capella recordings are not unlistenable, but definitely are not a strength. They are significantly heavier than I had expected them to be from pictures! However, comfort is excellent so long as your ears don't get heated. Denon includes a 3 button cable that is clearly intended for use on the go, but I honestly doubt you'll want to take them outside for a walk. If not because of the heat (sweaty ears are a blechy sitaution!), the weight alone will dissuade all but the most masochistic of headphone fans.
Probably the most interesting thing is how favorably the average music listener respond to these. I've given them to three coworkers of me, two males (Beats and Klipsch Image One users) who are absolutely bassheads with metal and rap, and a female who enjoys kpop. All three were thoroughly impressed by the D600 when I let them listen to them. Both males expressed an interest in purchasing a pair at the price I paid for them. The female, despite having listened to headphones I've brought to her such as the HD 800, the Paradox, and the LCD-2, believed these to be her favorite pair and the one with the best sound.
Overall, a reasonable purchase for bass lovers everywhere! If you have a chance to listen to them, do give it a go. I don't think you'll be disappointed.