I bought MM 550 about half a month back, I also have QC 3 Quiet Comfort 3 as comparison.
Sennheiser Travel MM 550
1. The sound is clearer than QC 3 even with bluetooth. Noise Cancelling is good enough but not up to my expectations. When I am driving my car, I will still be able to hear the humps and bumps that my car go over but it does cancels out the sound from my car engine. Sounds like children screaming, or any other loud noises can still be heard, which is normal for most noise cancelling headphones.
2. With the bluetooth, it's very convenient for me to shop, walk or do any activities without any wires being entangled anywhere.
3. The Talk Through function is also average, not able to hear clearly when the other party is speaking on a softer side.
4. Battery Life. If you are using noise cancelling plus bluetooth, the battery life is only about 8hrs, which is pretty short for heavy user like me at times. I am ordering my 2nd battery but will take abt 1-2 months to receive it where I am from.
5. Volume control is limited when using bluetooth, the volume will not be as loud as connected through a wire but that's the point of noise cancelling isn't it? Not to spoil our ear drums by increasing the volume to compensate the surrounding noise, so yes it's adequate for normal usage.
6. When the bluetooth is on, there is a slight ruffling noise on the background, I only noticed it when I am in a more quiet environment.
7. When talking in a conversation using the bluetooth, my voice can't be heard clearly by the other party, so for normal conversations it's ok but not for important phone calls.
8. Yes, the padding can be quite hot if you are not in a air-conditioned environment.
Overall, I like this model because of it's multi-functions and convenience. There's no comparable wireless headphones out there in the market as yet for now. I am looking forward for more high quality wireless noise cancelling headphones/earphones. If anybody come across any do share with me, thanks.