Having one or two sets of good headphones and to my mind a good ear, I thought I would add my contribution to the pot for what it's worth. I own Sennheiser HD25, Bose QC 15, B&W P5, yamaha 100 in ear, B&W C5.
My first comment is that 'good' sounding headphones is purely a subjective phrase and dependent on your ears and their ability to hear frequencies, and then personal taste. This ability changes throughout our life and different people hear different ranges of frequencies. For these purposes I can only speak for my hearing comparing the Bose QC 15 with other quality headphones. The Bose certainly have a laid back 'smiley' equalisation associated with them. I wouldn't call this bad, far from it they are open with a wide sound stage and due to not overemphasising the midrange sound in some way more realistic. The sound from bass to mids to trebles is seamless unlike some of the the other headphones I own. The bass in particular is articulate and does not suffer from muddyness and confusion with the midrange again suffered by other headphones that should know better. One of the most pleasant factors of the Bose is that you are not aware of the bass until it's there in the music. Yes the P5s and the sennheisers offer great mids and trebles but this can be very wearing over time and you do not get the "here in the room" experience that you get with the Bose, it feels like someone is singing / playing straight into your ears which isn't pleasant for some recordings in fact it can sometimes be painful if the recording is shrill. Having got sound out the way I will move to the noise cancelling part of the headphones. This puts a smile on the face of anyone who tries it for the first time, however can take a little time to get used to. The feeling is that of unpopped ears when no music is playing through them, however when the music begins this soon fades away. They do take some getting used to, you strangely find yourself turning down your music while you're out in the madding crowds, which seems odd but you find that you just don't need the volume. I was not aware until I did a side by side comparison of my other headphones how intrusive ambient sound is on my music and found myself hearing drumstick counting hits at the beginning of songs that I never knew were there, and all this in the middle of a big city! yes I admit it I am a big Bose fan but only because I tried them and liked them and not because of the name, I have tried other big names in headphones but their fatiguing in your face nature make them difficult to live with on a regular basis for me. These are just my honest thoughts, if you find them useful Great! If not please ignore them.