I have always been fascinated by noise cancelling (NC) headphones.  But whenever I tried them (perhaps for the first time decades ago and usually at the Sharper Image) I was always disappointed that they only cancelled low frequency sounds. They did nothing about voices or unwanted outside music.
 
A few weeks ago I purchased a pair of Bose Quiet Comfort 15 which was on sale to see what the latest in NC yielded. I wanted something to reduce outside noise, especially music, both at the gym and in the subway.  A week later I began reading about a new “IEM” model that Bose came out with last month that was supposed to be several steps forward in NC effectiveness.
 
Enter the Bose Quiet Comfort 20 (QC20) $300.
 
Features:
The best NC I’ve ever heard, much better than the QC15s in that department – now voices/and outside music are reduced to whispers. It has a button to let in surrounding sounds for conversions on demand which is excellent. It does work when battery dies (see cons). It comes with an USB charger cable, soft case and S, M, L size silicone inserts. Switches include an on/off switch on the thin battery compartment, an inline pause switch and an on-demand switch to listen to outside, surrounding environment. They are easily driven by anything I through at them and don’t seem  particularly amp sensitive.
 
Fitt :
They primarily sit on the ear canal creating a good seal in my case. There is a piece that fits into the curved section of the ear above the canal to anchor it in place. They have never fallen out or come loose. There is no pressure inside the canal that there is with the ETY 4S or the TF10pro. I notice they are there but without any discomfort. I could wear these all day. It is a brilliant design. 
 
Sound with circuitry engaged:
With circuitry engaged: When I a/b’d the QC20s with the QC15s at the Bose store I immediately could tell that there was something amiss with voices on the latter. The QC20s do not produce what I think is the typical Bose sound – big bass and bright highs with a missing mid-range.  These have a very full mid-range with a deep full bass that is not quite as well defined as the QC15s. Emphasis on the midrange is where it should be.  I would not call them dark sounding but their signature is closer to the TF10Pro than to the Ety4S. Organ music is very satisfying and can have the visceral sound of full headphones. The highs are a bit recessed; as such they do not have the last word on airiness. They do, however, give a good impression of the surrounding venue, if there is one.  I would not call them obvious revealers as I would the Ety 4S but everything I expect to hear (and then some) is there, just not in your face. Timbers are very good.  I have not found that they favor one type of music over another.
 
Sound without circuitry engaged:  Completely different and not so good. Not recommended.
 
Quality:
The silicone ear pieces seem thick and durable. They are reportedly hard to remove but I have not tried.  The cable is of excellent quality – I don’t think I’ll see any issues.  The soft, merely adequate case is a little too small.
 
Battery:
A small, thin rubberized unit. Not user replaceable. It has a listed limit of 500 recharges /16 hours per charge.  USB only recharge. Two green LEDs, one to indicate battery charge and one to indicate noise cancellation engaged.
 
Comparisons I've made:
                Bose QC15 (returned) – Universally acknowledged as having the [formerly] best NC which is not nearly as good as on the QC20. They still let the voices in. Very comfortable over the ear but can get warm.  Sound has typical Bose sound, mid-range suckout. One notices first the bass and highs but not much in between.  Highs can be a little harsh.  Take away the bass and the mid-range is thin and threadbare.  To its credit it has some of the best, tight bass I’ve ever heard on a headphone. For that reason it was tough to give up. But alas the bass dominates and over powers the Bose-sculpted sound. My ears hurt from the sound pressure.
                UE Triple-Fi 10 pro – My, how my impression of these has changed.  These now sound cloudy and darker by comparison to the QC20. They keep losing the seal and frequently fall out of my ear canal. Not particularly good at NC.
                Ety 4S – Still wonderful. Not a particularly relaxing sound – they make you sit up and take notice. Very balanced sounding but definitely not for listening to organ music. I am always aware when they are inserted with some cable noise. Very revealing. Passive noise isolation is very good, especially with the foam inserts but does not do nearly as well through the entire spectrum as the QC20s, particularly in the low frequencies. The QC20's have a much fuller, warmer sound.
                Stax SR-001 – Relatively flat sounding The SR-001 has one of the least electrostatic veils I’ve heard. It also fits on the ear canal like the QC20 but it uses a head band to press against the ear canal opening. This is not nearly as comfortable the QC20. NC is barely noticeable.
 
Cons: 
The battery is not user replaceable. USB only recharge. Bose has not yet announced a replacement program but an out-of-warranty replacement for the earphones is at 50% of the cost.  The Sound without active circuitry is muffled, and, well, not so good.
 
Summary:
I now call these my Sanity Phones. I can hear good sounding music in any environment and listen at sane levels.  I could use them all day if I wanted to because of their completely non-fatiguing sound, fit and noise isolation. Adding surrounding sound via the on-demand inline button is very effective, removing the necessity to remove the ear pieces when conversing. There is nothing offensive, physically or sonically, about them. Where they err they do so on the side of subtraction (slightly recessed high end). Not any part of the music stands out too much – it's a cohesive, relatively balanced sound.  My one concern is the non-replaceable battery and the not-as-yet announced battery replacement program. I would have preferred to use my own rechargeables.
 
Rating:
 
Highly recommended for their sound. Essential for their noise cancellation.
 
Associated equipment: Macbook pro, IMOD, Corda 2Move amp, HD580s
 
 

Excellent review with good sound comparisons that match my experience. I didn't realize the battery was only good for 500 charges though, with that issue I will likely use up the battery in mine in about a year as I end up charging mine daily as I tend to use them enough that the blinking light is on once a day.