Sonically, noise canceling circuitry screws up the sounds, muddles it. However, noise canceling definitely provides better noise reduction than soup imply noise-isolating.
I have etymotics er4, sennheiser 250, audio technical ANC. These all clearly outperform Bose sonically in everyday except for monoxide reduction. Bose Qc3 are dark, muddled, ill-defined and lack soundstage for the type of music I listen to , mainly classical and jazz. However, if you re in a situation where noise is that much of an issue, you may not be in an environment conducive to careful detailed listening anyhow.
I love the Eytymotics--soundstage and clarity are wonderful. However, they are quite unco,for table. I can tolerate them for 2hour plane trip, but not longer. They are so small, however, that I just keep them in a small pocket in my backpack so that I always have something decent to listen with in a noisier situation.
A very, very pleasant surprise came with the Logitech Ultimate Ears, which, In my opinion, outperform etymotics in terms of soundstage, definition, clarity. I know, I know... Logitech? But , yeah, they're really good. . They do a poorer job with noise isolation thatpn the ety's, but ate more comfortable.
The Sennheiser 250's are very good in terms of soundstage, definition, and clarity, and they are the only noise-canceling headphones that actually sound great musically and analytically. They are fairly linear and transparent. Noise cancellation is excellent, but their on-ear design makes them inferior to the Boses in terms of notal noise reduction. I really wish Sennheiser would make a noise-canceling around-the-ear closed back set of cans.
So in summary:
Noise reduction: etymotics > Bose > Ultimate Ears > Sennheiser 250
Soundstage: Sennheiser 250 > Audio Technixca > Ultimate Ears > Etymotic > Bose
Clarity and Definition: Sennheiser 250 > Ultimate Ears > Etymotic > Audiotechnica anc7 > Bose
I guess, what we're all saying by consensus is: Forget Bose, think Sennheiser 240 (or non canceling closed back if you don't need noise canceling) or earbuds (Etymotic or Ultimate Ears).