Pros: Detail, soundstage, looks
Cons: Comfort, extended break-in needed
I'll first say that when I initially got these headphones and plugged them in, I was pretty disappointed. The sound was thin, plasticy and terribly rough on my ears. BUT I had read enough about the K701's to know that people recommend an extended break-in period of at least 100-200 hours.
I made a temporary setup in the laundry room: old iPod plugged into old Onkyo receiver, K701's plugged into receiver's headphone output. I put a varied playlist on repeat, and kept the volume quite low at first. I let it run 24/7, each day raising the volume some until it was at a decently loud listening level. After about 11-12 days I had a listen - WONDERFUL! There was now a deep liquid bass where there was almost nothing before; tons of detail, crystalline highs, thick timbral textures on acoustic instruments, excellent vocals (especially female).
Be prepared to do something similar if you get these. If you're not willing to delay gratification by a week or two, you might want to get something else, but stick with it and you'll be glad you did. I suspect the people who hate K701/702 just didn't do their research. It takes a few hundred hours to hear the baseline potential of these headphones - it's a night and day difference - but after that they seemed to gradually improve in more subtle ways, at least for a few months.
I would describe the sound of the K701 as very neutral - they really reflect what you put into them. If you want heavy bass, you need to play music with heavy bass, etc. They are revealing enough to reveal poor recordings, but make good recordings sound wondrous. For this reason some people describe their sound as 'clinical' - I see why they say this, but these are also quite musical headphones in my opinion. As I mentioned above, acoustic instruments sound very lifelike. John Coltrane's saxophone is palpable. I'm quite able to lose myself in my favorite recordings, which says to me the headphones are not overly 'clinical.' I only deduct half a star from the Audio Quality rating because they are probably not quite as mind-blowing as something like the HD800 or other exotic boutique headphones.
Only big con to me is the headband. The middle pad, after more than half an hour or so, sometimes begins to feel like a half-inch-thick bolt being slowly threaded through your skull. I'm not sure why; it's thick and feels pretty soft to the fingers. It must just hit some kind of pressure point on some people. It doesn't always hurt but when it does I just stuff a wad of gauze under that spot - a minor nuisance that I accept. Otherwise, the comfort is great - lightweight, and the earpads are luxurious, though after a few years mine have lost their velour-like feel and are kind of faded in spots. Not everyone likes what could be described as the 'trendy ipod white' looks, but I think they're tasteful and pleasantly high-tech looking.
For a long time I listened with Emu 1212m -> Millett Hybrid Max. Recently I replaced the Emu with the Gamma2, which is even better.