The QC15s are a little on the large size. I like the bud-size of the ie2s. I can listen to them with my mp3 player on the bus, slip them into my pocket, carry them to school, wear them to bed and fall asleep listening to music. Everything. Versatility. They don't have noise cancelling tech, but they do drown out most things. Unless I'm waiting for the bus under the train bridge I don't really hear traffic... I can listen to music at 40% volume and still think it's loud, they've got the super awesome little rubber arms that keep the buds firmly in place in my ear with no wiggle (they also absorb some extraneous sound), they direct the music directly into my ear canal (is that what it's called? "Ear Canal"? The sound is directed right into my ears, where as, lets say the Apple headphones, direct sound into ones anus).
The QC15s look like they'd be good for something like a PC/Desktop set up - though the lack of a mic for game comms is a drawback. I like my headphones like I like my chances of getting laid - the smaller the better. I did try them out a while back, just a demo set at a store so I couldn't pick the music really, but they did sound nice. Shit, everything Bose sounds nice. I'd kill for a nice custom Bose gaming headset, something like the A-20. Eff Turtle Beach.
"War must be, while we defend our lives against a destroyer who would devour all; but I do not love the bright sword for its sharpness, nor the arrow for its swiftness, nor the warrior for his glory. I love only that which they defend: the city of the Men of Numenor, and I would have her loved for her memory, her ancientry, her beauty, and her present wisdom. Not feared, save as men may fear the dignity of a man, old and wise." - J.R.R. Tolkien