As for sound, let's start with the positives. The noise-canceling on the AH-NC732s is quite good--arguably superior to that of either Bose model. With the QuietComfort 3s, the loud air-conditioning fan in my office was muffled, though an audible hiss was left as residue. With the AH-NC732s, the faint hiss was still there but it sounded slightly more pleasant. Like with all noise-canceling headphones, the noise canceling does exert a slight bit of pressure on your eardrum. For many people this isn't a problem, but some people get wigged out by that pressure.
One thing the AH-NC732s can do that the Quiet Comforts can't is play music when the noise-canceling circuitry isn't engaged (if your battery dies in Bose headphones, the sound cuts out). However, if you turn off the noise-canceling circuitry (a single AAA battery powers it), the headphones simply don't play as loud and the sound comes across as a bit muffled. In other words, it's hard to listen to music without the noise canceling activated--but at least it's possible, which will be helpful if the battery conks out in the middle of a long flight.
Overall, we found the AH-NC732s to be fairly laid back. They're not as efficient as the Bose headphones, so you have to crank the volume on an iPod to equalize the headphones. Perhaps we expected more from them because this is Denon and we're used to having the company produce high-quality products. But the AH-NC732s were clearly beat by a pair of regular headphones, the Ultrasone HFI-580, and they lagged behind both Bose Quiet Comfort headphones. That's not to say the Denon's sound bad, but when you're dealing with $300 headphones, merely being good doesn't cut it.
The Ultrasone HFI-580s offered better clarity and more detail, along with tighter bass. The Denon pair also didn't measure up to the Bose 'phones in the bass department. Quiet Comfort headphones are known for their thumping bass (some would prefer it to be more restrained) and by comparison, the AH-NC732s just came across as sounding a little timid.