Because these are over-the-ear headphones that offer a tight seal, it can get rather steamy inside the cups, especially on hot days, although they breathe fairly well for such headphones. On long plane journeys, your skin will also get slightly moist underneath the cushions, so expect to take them off for short periods to give your ears some air. Also note that the 3.5mm cord is detachable, so, if you just want to block out the outside world and catch a nap, you can do so.
In terms of sound quality, the first thing we noticed about the 15s, compared with the 2s, was that the overall sound was smoother and more tonally balanced. With the 2s and 3s, Bose seemed to have messed with the bass, pumping it up (we found it rather thumping), but the bass on the 15s is tighter and punchier. Hip-hop fans may prefer the bass on the 2s, but that's a matter of listening taste.
The 15s offer excellent detail. When first using the headphones, we experienced a strong urge to relisten to our entire library of music -- a welcome symptom of acquiring high-grade cans. That said, the 15s have a closed-cup design, so the sound feels more 'stuck inside your head' than you'd get from open-backed headphones. As such, they don't quite measure up to some other high-end headphones that we've tested in this price range. Still, these are probably the best-sounding noise-cancelling headphones we've heard to date.
As is the case with virtually all of Bose's headphones, the QuietComfort 15s aren't exactly a bargain. But they're a good step up from the QuietComfort 2s and offer improved sound and noise-cancelling performance. Looking at it that way, we can at least say they're a better deal than their predecessors, or the smaller and similarly priced QuietComfort 3s.
Additional editing by Charles Kloet