Pros: My husband LOVES these!
Cons: Expensive, does not cancel any noise for me!!!
The bottom line: May work for some but I must have super-sonic hearing...I didn't notice any noise cancelling properties at all!
THE CLAIM
Acoustic Noise Cancelling® headphones Proprietary Bose technology electronically identifies and reduces unwanted sound while faithfully preserving the audio or tranquility you desire.

TriPort® acoustic headphone structure Exclusive technology features tiny ports in the earcups to produce a rich audio performance.

MY EXPERIENCE
My husband doesn't ask for much, so when he does I generally go out of my way to get it for him even if I think it's ridiculous. This is one of those ridiculous requests. Ridiculous because we rarely fly, perhaps once or twice a year, and beyond that we don't listen much to music at home and never through head phones. So when my husband asked for headphones costing $299 I did consider refusing him this once. But, I went back to my original thought & he rarely asks, so I'll make it happen.

The first decision was whether to buy the older model QuietComfort 2 or the new, smaller, and more expensive QuietComfort3 model. My first thought was the major reason he wanted them was for flying. He hates when seatmates talk to him, so the large QC2's would be a benefit here & he'd like the physical bulk of the QC2's. I'd also read several in-depth reports on the sound quality an many found the QC2's to be superior.

Given the price difference, the size, which in this case, the larger size was a benefit, and sound quality assured from the reviews, I ordered the QC2 from the Bose website. Lucky for me by doing so I was able to get six months same as cash and a free MP3 player.

Since this was a gift, I preloaded the MP3 player with a memory card and songs and presented the gift to my husband. The headphones were a hit. My husband has used them on many occasions since, many more than we thought he would. He's used them on several cross country flights and he found they worked well for closing out the crying babies and talkative seat partners. He's also used them at home when is partner (moi?) snored. I on the other hand have tried them and find them useless. I'm not sure how it is, but he thinks they block out most sounds, I find they block out nothing.

Perhaps my hearing is just better (and all this time I've been accusing him of selective hearing) or maybe it's better at blocking a level of hearing for him that's not blocked for me. All I know is that if I had spent that kind of money on headphones for me I'd have returned them immediately. They do sound fine, but not spectacular when listening to music, and remember that free MP3 player? He hasn't a clue how to use it and he prefers the silence anyway. Silence? I can hear everything!

They are comfortable - over the ear and more importantly over the head. He didn't want a behind the head style although when he does wear them to bed on those nights the *dog* snores, it's tough to roll over or change positions because they are so large. Still, he wouldn't give up the size or shape, for him it's perfect because he wants the physical barrier between him and anyone who would interrupt his wall of silence. :)

UPDATE JULY 28, 2007
The headphones are still holding out well after many trips and use around the house. My husband still insists they're the best purchase I've ever made for him and I still say they're they biggest Placebo ever invented. He absolutely will not go on any form of public transportation without them and I think putting a band-aid over each ear would be just as useful. I sincerely cannot understand how two people can have such a different experience with the same piece of equipment. Every time I come across the receipt for this incredible waste of money I just think the Bose people must be giggling to themselves and now when I fly and I see people wearing them I now longer assume they're flying in peace, I simply think they've spent a whole lot of money on some really big band-aids.

Oh, and the free MP3 player that I lovingly loaded with all our favorite songs still goes unplayed. Far to technical for him. :(