Hi,
 
I made a video review of Bang & Olufsen H6. Most of the important things are covered in the video so I'll keep it brief here. 
 
YOUTUBE VIDEO (BELOW)

 
BUILD & COMFORT
The overall build quality is great. It's sturdy but lightweight. The top part of the headband is made out of leather. The leather is not very smooth but it's very nice. The ear cup is also covered in soft leather. It is very comfortable, because it's light and doesn't squeeze your head too tightly. I can literally wear it for hours until I fall asleep. 
 
SOUND QUALITY
The sound is surprisingly flat, but a little bit on the warm side. The sound is not as airy as the open back cans, but the soundstage is very good for a portable closed back. The amount of bass is comparable to Beyerdynamic T5p. The H6 has a little bit less bass than the T5p. If you think the T5p is bass light, you will not like the H6. The T5p is also little brighter and more open than the H6. The midrange and high is very sweet and not tiring at all. I listened to the whole album of Comeblack by Scorpions and my ears don't feel tired at all. The separation between instruments is great even for a heavy metal album. I also tested it with Rachmaninoff's Paganini Rhapsody (played by Lisitsa). The detail is good but the soundstage is not that big. In my opinion, jazz is the kind of music where the H6 performs best. Try Nat King Cole's "L-O-V-E" or Zee Avi's "Siboh Kitak Nangis" and you'll know what I'm talking about. What I don't like about the H6 is the sensitivity. For a portable headphones, I expect the sensitivity to be a little higher. I couldn't find the actual figure on B&O, but I guess it's pretty low. This thing is not much louder than MrSpeakers' Mad Dog plugged in directly to my iPhone, and remember that's an orthodynamic headphones designed to be hooked up to an amp. The H6 sounds even better with an amp, but I'm not a big fan of carrying a portable amp around. The thing is, the H6 sounds pretty good without an amp, just not as loud as I expect it to be.
 
ONE SENTENCE COMPARISONS (Similarly Priced Portable Headphones)
Bowers & Wilkins P5  
- The leather is nicer than the H6, but that's it!
 
Beats Studio (2nd Gen)
- I turned on the Bass Reducer on my iPhone, it still got too much bass!
 
MrSpeakers' Mad Dog
- Sounds better than the H6, but it couldn't even fit my messenger bag!
 
Bose QC 15
- Inside an airplane is the only place it has the advantage over the H6!
 
Sennheiser Momentum
- If you have a large head like me, this thing is just not for you.
 
CONCLUSION
The H6 is not the best sounding headphones for $400, but it got the looks, portability, comfort, and build quality without compromising the sound quality. You can also crank up your iPhone to the max without going deaf instantly. For me it's definitely one of the best all round portable headphones out there.  

I've been looking for a over ear closed back headphones that I could wear anywhere for a while. My Bose QC15 is great when I take a plane or train (good noise cancellation, very comfortable), but the sound quality is rubbish. My Grado SR325 sounds amazing at home, but I couldn't even wear it on my bed because my girlfriend would complain about the sound leak. My Bowers & Wilkins p5 looks cool but sounds rubbish and uncomfortable because I wear glasses. My Shure SE530 sounds very good but doesn't have the slam and the airiness the full size headphones does. The Beyerdynamic T5P is just simply like having all of those headphones combined. The details is just amazing. I couldn't even believe a closed back headphone could sound this good. The price is pretty high, but it is still cheaper than buying 4 "one trick" headphones I mentioned above.