Hey yall. This is my first post. I used opinions of people on this site to make my recent decision to purchase the Senn MM 550 NC headphones so I felt obligated to post my own review now that I have them out of the box. I travel fairly frequently and I needed a decent NC headphone. Eventhough Bose seems fairly unpopular on this site many agree Bose posess the best Noise Cancelling technology out there. This was (and still is) the other set of headphones I am still looking at however I think I have found what I am really looking for in the Senn MM 550. Here we go: Firstly, I was impressed by the light weight and the comfort of the 550 was not bad at all. I thought they felt similar to the qc 15 in that regard from when I tested them in the Bose store. The 550s have immediate other physical advantages over the bose in that they have: Blutooth, tak through, ability to work without batteries and ability to control your mobile device with controls on the side of the headphones via blutooth. In terms of sound.... the Senn's sound amazing. They have this "SRS" function on the side that really works great. There is a whole 1 page description on how this works in the manual but lets skip all that and lets just say it works. Sound is clear and crisp in the highs. Lows are not muffled or over exaggerated. It left an immediate impression on me after having heard the bose in the store. Someone posted on another thread that they "didn't notice a difference" with the SRS on and off and weren't sure what it did. Well i can certify that there is something wrong with the unit they purchased or with their ears as the SRS drastically affects the quality and I got sound out of compressed music such as ACC's and MP3's that I didn't think was possible. I do not have a set of QC15's at home so to be fair I have not done a side by side (and I still plan to) but these 550's just made an impact when I first hooked them up that I don't recall getting when listening to the Bose QC 15 at the Bose store last week. I do have a set of Bose Tri-ports here and did a side by side on those and there is just no comparison in the sound quality department. Highs, mids, lows. The Triports have less impedance and are louder especially in the bass but not nearly as crisp and the mids are weird to me in comparison and highs of the triports are not defined or crisp like the Senn's. All that being said (and the Tri ports being an old model now) I am still considering purchasing the Bose and doing a side by side and returning the one I like the least. The next point is the Noise Cancelling. The Bose blow away these 550's for sure in that regard. The Senn's seem to do a good job on cutting out the humm type noises. I tested them next to a loud window AC unit and it cuts the fan noise well and also next to a large server rack. They perform well in those conditions but I don't think they will fare well against crowds of people or places with banging etc. I will be using them on a flight to DC tomorrow which will be the real test for me (I will let ya'll know). But the Bose cut out a lot more sounds and give you that total silence feeling. The 550's allow other frequencies through aside from that fan low umm noise. The 550's sound quality is also affected by turning on the NC fairly signifigantly in the low's. I didn't get to test this specifically on the QC15 yet. Cost is high for both the qc15 and the mm 550. The 550's have been replaced by the 550x and I got them for $310 at B&H down from $500. (difference between 550 and 550x is a newly added comm protocol -- see Senn's website -- no other difference) The blutooth function worked extremely well to my suprise. See the 550's come with both a hardwire option or you can use it blutooth. Even using blutooth the SQ of the 550's far, far surpassed the Triports I have here and also much nicer than what I remembered hearing of the QC15s in the store. I plan on using this feature a lot more than I originally anticipated as the SQ is certainly acceptable with my ipad (little quiet when I use BT with my iTouch for some reason by quality still good). In contrast, when using a cheap usb Blutooth xmitter from my PC, the SQ was unacceptable so this feature is going to depend on the device you are using it with. I'll need to purchase better blutooth for my computer. I find myself really using the buttons on the side of the 550's a lot to pause and ff or rewind. It is a nice feature when using blutooth. Ok I think this is long enough for my first post. Other key bullet points real quick: -- Bose single aaa battery beats the Senn mm 550's proprietary battery in life by double. -- Trade off between having to buy an expensive spare battery from Senn (50 bucks) and the convience of being able to charge the Senn battery with any mini usb charger which is nice. The usb plugs straight into the battery which is nice since you can charge a spare with no need for an extra charger. -- Carry case of the Senn is just flimsy nylon compared to the Bose semi-hard case. -- 550s have talkthru feature when NC is on. Kind of neato but not really that hard to remove the cans to talk to someone but the feature is there vs the bose where it is not. -- Sound Isolation is great on both (both are closed cans obviously). There is virtually no bleed on the 550's so you won't annoy the guy sitting next to you even on high volume. Didin't test with the Bose on bleed sorry. -- I touched on this above but the bose wont function at all when the battery is dead but you can still use the Senn with the provided cable when the battery is dead (although aaa batteries are not to hard to come by to get the bose up and running again) My preliminary conclusion is that if you want far superior SQ (even when using blutooth) with okay NC the Senn mm 550 is for you (especially if you can get them on sale like I did). If ultimate NC is your goal, you should try out the bose. That being said, I feel it is only fair to do more of a side by side so I still plan to either buy the Bose and compare or bring my mm550's into the Bose store. I love these 550's and Im pretty much 95 percent sure Im sticking with the Senn's but either way I'll let yall know when I do the side by side. Because as I said I am just really impressed with the sound of these 550's as their biggest plus of all. I hope this helps anyone like myself who is trying to decide on the best pair of NC headphones for them.