Read around on the internet and you will find a fairly reliable consensus emerging on Bose noise-cancelling headphones. Audiophiles tend to hate them, because they are expensive but have low fidelity (compared to headphones in their price bracket); people who buy them purely for noise-cancelling tend to like them, and even think that they sound pretty good.
Audiophiles who want noise mitigation tend to go for the passive attenuation used with IEMs (in-ear monitors, like the sort that performers wear on stage). I have some Shure IEMs, and they are very good at blocking out external noise, but at the cost of inserting something into your ear canal. Worse, in an environment such as a plane, you are often interrupted and have to pull them out of your ear canal, meaning that you have to keep struggling with refitting. (Yes, you can get a microphone unit to avoid this, but that's pretty inconvenient in itself.) Let's not even talk about earwax!
The Bose headphones are very easy to move and replace, so what you lose in absolute attenuation is more than offset by convenience.
The attenuation itself is very good. Without noise cancelling turned on, the headphones do a good job of muffling the external world, but when you turn it on there is a clear "silencing" effect. I haven't tested other noise cancelling headphones, but from reading reviews I'd say that it's pretty clear that Bose are technically the best and - even though these headphones will not silence the external world or even come clear to it - once you actually start playing music your brain tends to go along with the process and exclude any distracting noises that might make it past the active cancelling.
It took me a while to get to try these in a plane, but there is absolutely a reason why the QC15s are so popular with air travellers. I always assumed that the audio output from plane seats was very poor, but once the environmental sound has been reduced from a rumbing bass roar to a comparatively benign mid-level hiss you can enjoy a film far more ... or your portable music player, if you'd rather. For a substantial part of the flight I was completely unaware of ambient noise.
Note that the lower listening volumes possible with these headphones, together with the passive attenuation in the earpad designs, means that these headphones have very little leakage. They are certainly kinder to other people in the room than any other headphones I have used.
BUILD QUALITY & COMFORT
Bose are often criticized on build quality: certainly the QC15s are very "plastic" in feel and you may be disappointed compared with some other headphones in the price bracket, which employ expensive or exotic materials. The "leatherette" headband and ear pads are functional, no better. My AKG K701s, which cost about two thirds of the price of the QC15s, look and feel like a much more expensive headphone.
That said, the QC15s are very light and surprisingly comfortable. Rather than being directly above the crown of the head, the headband is angled slightly forward, meaning that they are not going to move about when you lean back in your seat, and although the clamping effect against the ears is strong enough to result in hot ears, it is perfectly comfortable to wear spectacles at the same time as these headphones.
There are also some nice design touches in the QC15s. The battery compartment is very easy to access (yet secure) and takes one AAA battery (unlike the QC3s which used a special rechargeable battery). The cable snaps into the headphone on a special proprietary coupling. This means that the cable should be fairly easy to replace if it becomes damaged. (In any case, in the UK Bose give you a second cable included in the price: it's the cable for controlling Apple devices, which has a microphone on it, a volume switch and a pause/answer button.) The cable also has a slider on it so that if you are using the headphones on a plane you can set it to a higher resistance in order to compensate for the high gain circuits used in flight cabins: the difference is not enormous, but it is definitely helpful. Oh, and the cable is very light and straight: you might feel that it is flimsy, but actually I think that a light cable is very practical.
Stated battery life from one AAA battery is about 30-35 hours. You can't run the headphones at all without battery power, so battery lifetime is important. Fortunately, the operation light on the side of the headphones gives you very generous warning when the battery is low: it starts flashing when you have about five hours left. Some reviews have pointed out that it would be nice to be able to run the headphones passively without engaging the sound cancelling circuit (for example, for listening at home in a quiet room) but my feeling is that, in the real world, you are unlikely to be caught without a battery.
The QC15s come with a clamshell case that is really quite nice. The ear units fold flat against the moulded inside of the case, and it seems to offer enough protection that you could stuff it in a bag without worrying, although do bear in mind that overall the headphones in their case are a similar size to a hardback book, so they do take a toll on space in your hand luggage.
The QC15s are not neutral: they can sound airless in the highs (although this may be a feature of the noise cancellation itself); they sound slightly thin (I could almost say "tinny") in the mids; they boom in the lows, giving a likeable but not natural bass, and their bass extension is okay, but nothing special. You'll notice their weaknesses most on drums and acoustic or electric bass, which can have decent volume and boom but lack the impact and weight that you would get from a really good headphone. That said, the stereo image is good and the QC15s can be quite impressive on music that has sustained bass notes: what you lose out on attack is made up for on sustain.
You can compensate for the lack of punch by turning up the volume, but I don't recommend this. These headphones drive very easily and can be played at absolutely deafening volumes, but (in addition to being very bad for you) this is completely contrary to the entire design: these are intended to be listened to at comparatively low volumes, where their sound characteristics makes them a natural "loudness" circuit. If you listen to the QC15s at (very) high volumes the sound gets very muddy and uncontrolled.
Although it may sound as though I am really down on the sound quality, if you listen generally to popular music, classical or jazz, these headphones are easily good enough for dedicated home listening. They definitely struggle on hip hop, where their bass coloration and lack of real weight clashes horribly with the artificial EQ on that music. To be honest, though, unless you live on a constant diet of bass-intensive music you will be fine, and many owners will be trading up from headphones that may sound nowhere near as good as these do.
One big surprise with the QC15s was how good they are for surround sound. For interest, I tried the QC15s out of a Creative DDTS-30 (which is a little pocket-sized decoder that can provide a Dolby Headphone output) and they do a wonderful job with DTS sound such as the Steven Wilson mix of King Crimson's Starless and Bible Black. The colouration of the QC15s is also spot-on for gaming, with a lot of volume and a very good "out of the head" performance. It's a rather niche use for Bose headphones, but definitely worth trying.
I also tested these headphones with an iPhone and the Apple-compatible cable and was impressed how well the QC15s handled a phone call: the two people talking in the room with me were nicely "muted", the headphones were much better than the iPhone's built-in speaker at reproducing the voice of the person I was talking to, and the in-line microphone seemed to do a good job at picking up my voice. I don't have much use for this aspect of the QC15s, but it's something to bear in mind if you have a need for this functionality.
You should also note that I did listening tests for this review against the AKG K701s out of a headphone amp in a quiet environment. The K701s are a lot more difficult to drive than the QC15s, so if you are using headphones primarily with, for example, an iPod then the QC15s will cope a lot better than many "hi-fi" headphones.
These headphones are designed to be used in noisy environments, whether that means a plane or just a living room where someone is watching a television. As such, it isn't fair to complain that they don't have the same sonic fidelity as other headphones at their price point; the simple fact is that the Bose will sound much, much better than a high-fidelity headphone because you can hear them much better at tolerable volume settings. These headphones are absolutely not "jacks of all trades": they serve a practical purpose and the price premium that they carry reflects how well they do that.
There seems to be a lot of room for improvement in the QC15s but, at time of writing, they are pretty clearly the class leader for what they do and I didn't want to subtract a star since (on a major investment like this) most readers would be seriously put off by the suggestion that these are anything less than a five-star product. Bose has got the key things - noise cancelling, comfort and technical design - right.