I've been using the Bose QC20i for a couple months. The bose earphones are comfortable to wear for extended periods, I don't notice them unless I think about them, even after extended periods. I use a similar pair of bose athletic earphones when I run, same story - comfortable and secure.
All noise canceling headphones have a slight hiss when used. The newer QC20's seem to have less hiss than my old pair of bose on ear from way back, but its still there. If I'm listening to music I stop noticing it.
The active noise canceling doesn't eliminate all outside noise, but its very good at what it does eliminate. Anything that is "background" noise seems to disappear. Loud voices, taps, doors slamming will still be there. If you're listening to music you don't have to turn the music up loud to cover the background noise but you will still hear the music clearly and it will cover a lot of the other noise, like voices, even at relatively low volumes. I find that I can't hear the stewardess on an airplane at all. It doesn't block all noise but with a little music it reduces it below the threshold at which you notice.
The QC20's have a button that allows you to turn the noise canceling off. The music doesn't sound as good, but its useful if you're walking near traffic or listening for your flight to be announced. They will also work as headphones with the battery dead or turned off. Sound quality isn't very good without power, but they do work.
They're great for phone calls, say in a busy hallway. They allow you to concentrate on the voice of the person you're talking to and I've never had a complaint as to how I sound.
Reviews say that the mid range sound quality is good (not great) but the upper and lower ranges are a bit weak. I'm not an audiophile and I'm not sure I could even tell the difference. If sound quality is that important to you then these probably won't work for you.
Anyway, unless you are extremely sensitive to noise or a serious audiophile these will probably work great for you.
Also, customer service from Bose is great. I recently had a several year old pair of bose earphones,that the volume switch quit working on, probably because they got wet from sweat too often when running. I called bose customer service about it and even thought they were way out of warranty they took them on trade for a new pair of athletic earphones at 1/2 off.