Personally, I feel that this is the best closed back all-rounder I have tried for pure music enjoyment.  It honestly isn't perfect, but neither are any of the other few dozen closed back portables I've tried.
 
Style: This point isn't one I particularly care that much about, but the B&O H6 is one of the most beautifully designed headphones I have ever seen.  It's absolutely stunning to look at.
 

 
Build Quality: I simply can't fault the build quality at all, leather, metal and a little plastic which is some of the best quality plastic I have ever seen.  These headphones are built extremely well!
 
Comfort: The H6 is surprisingly very light! The ear cups are made of a soft lamb skin leather and have a huge opening which can accomodate my larger ears (the Momentum 2's and most other headphones I have tried are too mall for my ears).  The headband is so light and well designed that I honestly can't feel it at all, and the clamping force is just right such that it doesn't hurt or need stretching at all.  One thing that's also impressive with the lamb skin ear pads is that they sweat a lot lot less than other headphones I have tried.  Overall, I would say that this is one of the most comfortable headphones I have ever worn (and that includes open back headphones I have tried too).  I can literally wear it all day without a problem at all.
 
Accessories: The headphone's cable is a lot thicker than my Momentum cable and I really like its quality although the remote control buttons feel a tad cheap.  Sadly the headphone only comes with a carry pouch, albeit a very very nice pouch, but I would have loved a soft case for this price.  One other issue I have is that B&O charge a huge premium for replacement ear pads, fingers crossed these last long.  I think that sticking to the black colour is the safest bet to ensure that the ear pads last longer.
 
Sound Quality:
 
* Bass: The bass response on these headphones extends all the way down to 20 Hz and picks up every last bit of detail in the bass region.  The bass is a little elevated but not overly so, it gives a wonderful warm and punchy feel to tracks and I would confidently recommend these headphones for EDM as well as acoustic genres.  My only gripe here is that the mid bass (say at around 300 Hz) is pulled back which makes songs sound a little hollow, but at the same time, it gives the bass region less bloat and makes it more focused.  A bit of EQ can compensate for this if you find it a problem.
 
* Mids: The mids are pulled back a bit and not to the degree of something like the ATH-M50s.  Vocals still remain moderately upfront but are extremely smooth and not harsh at all.  The mids are present, but not harsh and vocalists still feel very close to the listener.
 
* Highs:  This is one of the few portable closed headphones I have ever heard with a fully extended (not rolled off) treble, but the treble remains extremely smooth and never strident.  I would say that the highs are some of the best I have ever heard in a closed headphone.  They do lack the sharpness and precise sound of my Audeze open EL-8s, but that's an open planar magnetic, so it's hard to compare them.
 
* Soundstage: Quite simply the widest and most open sounding soundstage I have ever heard in a closed headphone, beating many open headphones (like the HD600).  The soundstage makes these headphones extremely engaging.
 
I will admit that these headphones do seem to benefit from a really good audio interface output (e.g. MacBook Pro) and sound a lot better through this means than they do through a smart phone.  An amp improves their sound a little further too.  The H6s are definitely harder to drive than various other portables, and even some open backs.
 
Overall
 
I honestly can say that this is my favourite portable closed back headphone at this time.  I have tried several dozens of closed headphones from companies such as Focal, PSB, NAD, Audio Technica, Shure, Oppo, Beyerdynamic, AKG, Sennheiser, Ultrasone, Bower & Wilkins and various others, and overall can say that this is my favourite of the bunch for pure music enjoyment, comfort, design and style.  I would rate these above the Oppo PM-3 too regardless of the fact they are planars, the H6 has a far wider soundstage, more musical sound, more extended treble and far more present bass.
 
I would like to also mention that B&O stores are a great place to listen to these headphones but not buy them.  They only sell at RRP and are their staff (although very pleasant) are not at all technical.  The stores actually feel a little pretentious and snobby.  I would like to see B&O appeal to the audiophile and common man in their stores, instead of focusing on the headphones as a high end fashion statement.  Your staff should know the products much better than they do on a technical level and be ready to talk "geek" with those that are interested, aspects such as amping, sound differences between models, specs and so forth.

I recently bought myself the Momentum 2.0 over-ears and have a lot of good things and a few bad to say about them.
 
I really love their build quality, the feel of the buttons on the provided cable and the excellent case provided.  The leather pads smell and feel wonderful to the touch and it's clear that Sennheiser have not held back on the best quality materials here.  I feel comfortable travelling with these and listening to them everywhere I go including the train, work and walking.  The ear cups are still a little small for me and slightly touch my ears which gets uncomfortable after an hour or so.  I do usually have to give my ears a rest to let the discomfort go away.  They also clamp a pinch on the tight side which I presume may loosen over time, but mine hasn't yet.
 
Overall, I personally feel that these headphones shine with electronic genres and a lot of modern pop which has a lot of electronic elements.  The bass response is extremely solid and just the perfect amount for EDM without being over the top.  I just enjoy listening to dance music with these headphones, they have this wonderful punch and extend extremely low such that you feel the rumble of any sub bass.  I really love the mids too, they are really neutral and not recessed so vocals and instruments are represented very clearly.
 
My biggest gripe with their sound signature is the treble.  It's really laid back and rolled off such that you really lose clarity.  I find that this is most important for genres that use a lot of acoustic instruments and real drums such as country.  I can't say that this headphone would be my first choice if you're primarily listening to such material.
 
The soundstage of these headphones is good but not great.  I have heard closed cans (SRH-840, BeoPlay H6) with wider soundstages, but the Momentums do have a decent width which works really well for music such as EDM where you are mostly focused on the centered kick drum and sub bass in the tune.
 
Overall, I recommend this headphone with a few caveats; do they fit your ears comfortably?  are you OK with a rolled off treble and slightly more narrow soundstage?
 
For a good comparison of a headphone that has a more open and sparkly but less punchy sound to these, check out the BeoPlay H6s which are much more mid and treble focused with a less present but tight bass.