I've become a big fan of wireless headphones over the years with aptx codec, bluetooth 4.0 technology, and li-ion batteries becoming the standard. This allows more of the battery to be dedicated to driving the headphone rather than maintaining a connection. 
 
My previous wireless headphone the Sony MDR10RBT, was recently highjacked by my girlfriend, so I took the opportunity to look for an upgrade. I stand by the Sony being a great headphone at that price, just a good ol' walkin' around headphone, with not much to worry about except the music.
 
I've gone into detail for notable features, expect the rest to perform as expected respective to its rating.
 
Overall
 
I've found the M2 to be a great listening experience. I've noticed the momentum is a love/hate relationship with some, and if you're horribly picky about sound signature being perfectly dry and neutral this might not be for you. I would call this phone a music listeners headphone. I would never use this in the studio for any reason, but I use it whenever I'm not at my desk, and even at times over the TH-900 if I know I'm going to be getting up a lot. If I really need to focus on some work, the ANC does it for me.
 
Like the TH-900 I often find myself when one of my favorite tracks comes on, especially getting into the complete sound this headphone offers without taking anything away. It's easy to get lost in the entire track, and it's easy to pick out certain instruments if you want. Neither is so obvious and that's what makes this headphone special and worth it to me.
 
Sound Quality 4 / 5
Sound Signature: Depends on Individual, for me 4.5 / 5
Highs: 4 / 5
Mids: 4 / 5
Bass: 4.5 / 5
Staging: 5 / 5
Instrument Separation: 5 / 5
Comfort: 5 / 5
Build Quality: 5 / 5
Controls: 3.5 / 5 (Due to Next / Previous track being a bit unintuitive)
Value @ 350 USD: 4 / 5
 
Sound Quality
 
Very good detail across the board.
 
Sound Signature
 
Mostly balanced with seemingly emphasis on bass due to the excellent bass on this headphone.  Those who are extremely into neutral headphones might say that these are coloured. They are not studio monitor-like in any sense. The bass quantity + quality really is something to be treasured that a lot of cans really have a hard time finding this kind of balance with the rest of the sound in mind.
 
The headphone offers a smooth, detailed sound that allows your listening preference to stand out. If I want to pick apart a song, I can. If I want to lay back and enjoy the music, I can. Both seem to come effortless.
 
As an owner of the Fostex TH-900, I've come to truly appreciate any headphone that can have great bass without taking anything away from the rest of the spectrum, and this is a similar feeling to that, scaled down respective to the price.
 
Controls
 
The controls are for the most part very good, I have two gripes though. On my much cheaper Sony wireless, there are separate up down controls for both volume and track, while you press in the track switch button to pause. For the senn there is only one switch for volume and changing tracks. For pause, press once, for next track twice, previous track thrice.  With no tactile feedback on the button, it can be hard to tell if your press is being registered. It's easy to accidentally pause while switching to next track, and when some of your music starts tracks with silence this can take a second to figure out.
 
ANC can't be turned off which, if you're walking around and looking at your phone for example (I use my ears often in these situations), the ANC can leave you totally blind when you might otherwise not want to be. The ANC itself is very good however, and works as well as I could expect.

I'm well aware of what head-fi thinks of wireless audio technology but what do we have to lose from a ~200 USD phone that is wired(and sounds good wired) and also happens to be wireless?  Be not afraid.
 
You get the option of great sound when you want it and also convenience when you want to be wire free.  Aint nothin bad about that.
 
Disclaimer:  I typically don't dabble in closed phones because there has never really been a need for it.  I've owned my pairs of closed phones but mostly in the mid-price range, right around the range of the MDR-10RBT.  This is a price range that I am the most familiar with when it comes to closed phones.  This review is relative to the other closed headphones between $100-300 USD.
 
I originally felt this set of Sony's was going to fill a niche purpose in my life.  That is for use in the school's library because IEMS make my ears sore after some time.  I really only like IEMs for cycling.  Even more so I bought these for watching movies during the times I can't be loud.  It fills most of these purposes exactly to function and with ease.  But that's not why I spent the money I did.  You can get terrible sounding wireless phones for pretty cheap.  I was going for real hifi sound combined with that convenience and for the price, the MDR-10RBT delivers as well wired as any other phone you might expect to pay as much for, and passed my expectations with how much I've seen people around bashing on wireless tech.
 

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About bluetooth wireless:
 
It's imperative that an aptX transmitter is used with headphones that support it.  I would not suggest getting a phone that doesn't support aptX.  I am now aware of the difference in sound between bluetooth codecs and how the technology is evolving and that difference is significant.  aptX is a supposedly lossless bluetooth codec(I will leave that argument for elsewhere).  Through aptX the sound was while not on par with through a dedicated dac/amp, but still very good and well voiced.  I think a lot of the criticisms that wireless technologies gets can, at least partially, be attributed to the built in amp/dac that is in the actual earcup.  While some of it might be bluetooth, I saw no immediate problems with the sound while staying in line of sight of the device and within 20-30 feet.  With that said, I see a clear market as bluetooth and wireless audio technology evolves and I feel that bluetooth headphones have the potential to sound very good indeed.
 
I would also like to point out this.  I have had a lot of troubles searching for a reliable aptX transmitter to hook up to my tv.  Apparently this is not a device with very much demand.  Phones are the main source of aptX transmission which came as somewhat of a shock to me.  I'm still on the search for a non-phone transmitter that is worthy of my expectations.
 
Bluetooth range is kind of hard to rate because it really depends on the transmitter.
 

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Impressions:
 
The below impressions are with my home setup through Resonessence Concero HD, and either Decware Taboo II or Bakoon HDA5210mk3 amplifier.  Ratings are relative to the price of the headphone.  Consider the wireless sound on par with these impressions, except with slightly less fidelity due to constraints stated previously.
 
Highs:
 
No noticable sibilance.  Reasonable extension.  Not very much air as expected from a closed phone and comparable to my most recently listened to senn momentum over ear with a tinge more clarity.
 
7/10
 
Mids:
 
The highlight of this phone.  Very good and clear midrange, leans towards warmth but mostly neutral.  There is a sort of inviting/smooth sound to the mids that people would associate with warmth.  I would not quite call this a warm sound but it leans in that direction to my ears.  +1 over the momentums for me.
 
8/10
 
Lows:
 
The upper lows are not very offensive but can sometimes bleed a bit into the midrange on bass heavy recordings.  Some people may enjoy this sound but I am unfortunately spoiled by my home rig.  I am also the kind of person who prefers less bass than a little bit of mud and louder in the mix.  This is one reason I generally prefer the sound of IEMs.  A lot of them are tuned in such a way.  So I am being a little bit subjective compared to the rest of my review with the above.  The deep bass has some issues though.  It can be a little flabby and bloated which I think to some extent is to be expected in the ~200 range.  I still like this bass better than the audio technicas that I had heard so long ago.  I know the price difference is significant but they are a phone which many people have heard.  From memory I would think the momentums had an upper hand on the low end.
 
7/10
 
Staging:
 
Quite good stage width for a closed phone.  Depth I would consider average for the price range.  Placement of instruments is exceptional for such a headphone.
 
9/10 for the width, 8/10 for depth
 
Overall:
 
8/10  Very Good wired headphone, good wireless sound, and great conveneince all for a reasonable price.  No bashing needed.

This was the first piece of audio gear that I ever owned when I was venturing outside of what I could get from local electronics stores.  I wanted to see if this would provide any difference with my RE-0 earbuds and my friend's ATH-M50.  I cannot speak for what I know now which is a lot more, but at that time I felt it was an improvement over generic computer audio.  My friend and I were on a road trip, and this was the perfect device to use in our hotel rooms out of my laptop.  We were doing a lot of turntable.fm sessions then and I will say it performed nicely.
 
From what I remember to what I know now, this is probably slightly less than I would expect from a 50 dollar product.  There is a nice warmth going on and it is a bit grainy but it has a charm to it.
 
A few reviews I've read said that the fact that this dac only outputs 16/44 is a compromise. I don't think that's true.  16/44 is fine and will offer all the musical enjoyment you could ask for.  Once you forget about your file's sample rate and listen to music.  If you're looking for a touch of power and detail compared to your PC, this is for you.  If you have reasonable headphones you will notice a difference.  I don't recall any hiss from my experiences, but I don't doubt the other reviewers.
 
If you're looking in this price range and you can deal with the drawbacks, I think this is a pleasing product to have by your side. I've you've read about the hifiman house sound this kind of goes in line with that.  Happy hunting.