I've been using my K495NC for a month now, so I would like to add my impressions on this thread.
Much like many have said, it has a bass heavy signature with decent mids and highs. Out of the box, I found the highs to be slightly muddy, but I believe it's starting to become more pronounced. But regardless, it's a fine pair of headphones out of the box. It's clear, the bass is tight and the general sound has a lot of impact, which is what I bought it for.
I saw a test someone did on a plane, comparing the 495NC with the Bose QC15s, Beats Studio and a few other NC headphones and IEMs. It seemed to work wonders in that video, but when I test it in real world situations, I realized that there needs to be a more comprehensive test then just on a plane for those who want to use NC headphones on normal day to day public transport.
On the bus & train: The low hum of the engines are totally cancelled out. Voices can be heard if loud/close enough, and the higher pitch whine from the engines can be heard. This is without anything being played, just with NC on. The sounds, while they can be heard, are very soft and can be easily drowned out by most music at a reasonable volume.
Walking: Nothing except voices loud/near enough and the the vibration of your own steps. If you wish to cancel that noise, know that it's you, not the headphones! I remember reading about someone complaining about that; the same thing would happen if you wear IEMs.
The build quality and materials have been talked about pretty much everywhere, so there's no reason for me to talk about it. Instead, I will point out some minor issues I have with the headphones:
Clicking noise - Msninja said that he noticed some clicking noise when putting it on. It's not from any of the parts, but from the thin spacing in between the pads. There's some excess glue in there somewhere, and it's sticking to the pads when they are pushed down (when putting on), and when they reform (when settled on the head), they "unstick", creating those noises. I'm guessing this will eventually wear out and the sound would be gone, as it's starting to happen, but it can be rather irritating, especially when you first use it.
L/R lettering - I know many people praise this, but I just didn't like how the look is. It looks... cheap compared to the rest of the headphones. Perhaps someday they will rub off and look more faint, which would look better.
Cups rubbing against each other - Not really a major issue, but when you take them off and set them down, they cups are pressed together, and due to the weight, they tend to rub against each other. The black plastic are slightly scuffed because of that, but it's almost unnoticeable.
Cup placement - An issue I quickly identified with headphones without a bowl is placement becomes very vague. It took me a few days and switching between the 5th, 6th and 7th notch repeatedly, trying to find the best position of the cups and on my head. In addition, if you do not place the cups right or if the band is set too short, the cups can hurt as the pressure is placed all on your the temples (above/behind your ears), and the sound doesn't sound as good and NC doesn't work as well. So if you get a pair, and your head starts to hurt, it probably is because you didn't adjust it to your head right.
Weird sound when turning head really fast - Of course you wouldn't be doing that on a constant basis (and if you are, drugs are bad for you!), but on the occasion you do, you will notice a faint sound similar to a "WoowooWoo" when you do so. I believe this has something to do with the NC technology.
Case doesn't quite fit - It looks gorgeous, but I always have problems trying to fit the headphones in. Also, being the neat freak I am, and knowing that a loose piece of hard plastic/metal can do terrible things to seemingly sturdy things, I never did try putting in the charging adepter into the case. If anyone has done so and even after a number of violent shakes, the headphones come out pristine looking, do let me know
All in all, this is a great pair of headphones that is worth is price, and while I did list a number of issues, most of them have either been rectified or are minor enough for you to forget about.