The sound quality on these headphones are amazing! However, here are some things you should know about the M40x's before you buy them:

Pros:
* Tuned flat (all ranges the same, not flat as in pitch, of course), so you should be able to hear all sounds really well
* Can get loud even powered off of motherboard audio.
* Over the ear, studio look

Cons:
* Clamping force is strong
* Super floppy, sometimes hard to control
* Can get a little hot

Things to consider:
* Closed back. Others can't hear you, and you can't hear others. Limits sound stage, but this is a per-situation thing to consider.
* These are studio headphones, so the clamping force is needed to prevent audio leakage both in and out of the headphones.

Personally, I love them, but your milage with these headphones may vary!

Other notes:
* You can make these Bluetooth compatible with [1] and [2] (probably [4] as well)
* You can make these a gaming headset with [3] and [2]

[1] https://www.amazon.com/Bluetooth-Receivers-TaoTronics-Hands-free-Cancelling/dp/B0728GP9XH
[2] https://www.amazon.com/Josi-Minea-Converter-Headphone-Earphone/dp/B01GD3OOXQ
[3] https://www.amazon.com/V-MODA-BoomPro-Gaming-Headset-Headphone/dp/B00BJ17WKK
[4] https://www.amazon.com/L16-Stereo-Audio-Cable-Connectors/dp/B00KL7C10O

Ok, lets start - this review is purely based on audio and built quality. If you are about fashion and fancy names, than look elsewhere.
Last 15 days I was camping in MediaMarkt and Saturn (Deutschland) with one mission only - to decide which headphones to buy.
My budget what 300 euros. I have long experience with professional audio equipment, but very low or zero about headphones for listening and enjoying music.
I own Bose QC 15 and I like them very much, until today.
Music I have used for testing was various - from heavy bass based house, rock, vocal, live concerts, jazz, etc ... you name it, I tried it all really, and mostly from mobile devices - Samsung S4, LG Nexus 4, BlackBerry Bold 9770, Apple Shuffle and Nano.
Also, I was using devices in the store, to compare it. All music whas in mp3 format, 320 kbps, ripped from original CD's, audio quality aint something amazing, but still good.
I've definitely learned one thing - in a world of a high quality headphones, you REALLY REALLY DO NOT get what you paid for.
I was amazed how expensive h.phones can have poor sound. Ofc, at the end, it all comes Individually what kind of sound you like.
I have looked all the possible reviews, tests, advice's by professional audio testers, and so on. It can help, but also can be confusing sometimes.

Simple - Audio Technica M40X simply and easily outperformed 90% of the headphones in these stores. Remaining 10% are debatable, and questionable. I am talking about balanced and neutral sound, and they are all about balance, with a very, very balanced price, if not damn cheap for what you get.
Many h.phones are designed with lower impedance so mobile devices can run them easily, with louder sound. Majority have tweaked sound, for commercial, every day use. Accent on a heavier and punchier bass is everywhere, and on most of the models I've tried.
So you must be clear with yourself - what you like and want, what kind of music you listen mostly, mobility, quality, etc ...
Atm, I am testing them with "Greyhound" by Swedish House Mafia = bass is very, very good, deep, precise, and punchy. Almost perfect, exactly the way I like. Trebles are beautiful - crisp and crystal clear, middles and lows are marginally a little bit lower but mostly because I am trying to be very critical with them. Vocals are amazing, acoustic guitar and jazz sounds simply - brilliant. Some things with my Bose QC 15 are definitely can not be heard without amplifier and higher volume.
While we are talking about Bose, I have tried them all - QC 25 sounded very nice, but bass was over-dominating the whole sound experience and AT 40x had crispier, clearer sound stage. Active noise canceling is where Bose shines, but I've never liked that technology, I did always felt some sort of claustrophobic having them over my head.

At the end, my narrow choice was Sennheiser Momentum on ear. They are on special price at 139 euros in Saturn, and I have to admit I was very close to buy them. They look beautiful and sound was really classy. Bass was all over you, but very well controlled and balanced and the whole sound stage were impressive. More I was listening, I was getting more in love with them. But somehow M40X sounded the way I like more, plus Momentum is on ear headphones, and comfort was my main concern. They were very comfortable but still I started to feel some pressure over my ears after less than hour. But they sounded damn good.

M40X build quality is very solid, I have seen many models with double the price tag and they did not have this quality. They are built to last, by all means. Comfort is where Audio Technica have improved a lot on all "X" models and they are light and feel great. Included cables looks like they are literally indestructible - top quality. I was wearing them for over 5 hours and still enjoying.

At the end, and in my humble opinion I got this experience:
Audio Technica m50x - big brother, a bit pricier, have overall better sound with a little bit heavier bass, but still M40X have more neutral, better sound (at least to me)
Sennheiser HD 518 = good, but not that good, same price range so no reason at all to buy them
Sennheiser HD 558 = very good, overall experience is great, but M40X outperformed them, especially on highes.
Sennheiser Momemtum over ear = hefty price tag, but I really could not find how and where they are better. More I was listening, I got more determined to avoid them. Your call, but price versus quality ratio is not justified by all means, differences between them and M40X were marginal.
Sennheiser Momentum on ear = my next favorite. Deep, heavy, beautifully funny bass with great overall sound stage. Cool look.

Beats = Avoid, avoid, avoid. Please, do yourself a favor and save money, all of them are all about bass, bass, bass and after 2 minutes you felt like your brain will explode .... sound stage was poor, overall experience disappointing. Highly not-recommended.
Bose = sorry to all Bose lovers but except noise canceling (where Bose is the champion) I did not hear a single model that have sounded better. I really tried and even own QC 15, but M40X was a clear winner, and way cheaper. QC25 was really good.
Sony did not have anything serious there to match M40X. I have used before 7560 and M40x are better.
Philips comes with fashion, but Fidelio X1 comes also with ridiculous price. Although performance were really good, but again we are talking here about small differences, very small.
B & W P3 = very, very good headphones. Was a pure pleasure listening them, overall great performance but they come with double the price. Best portable headphones here, and they look amazing, but very pricey.

There are infinite numbers of headphones models there but I was focusing mostly on what was recommended by users and experts. Many recommended models where not available in these stores, but I had pretty wide selection and some of the top and best sellers where there.

So, lets try to cut the story here with the conclusion: Audio Technica ATH-M40X are probably the mostly overlooked headphones on the market due to big and very popular brother m50x.

Overall great sound stage, neutral, amazingly balanced and warm deep sound with strong, precise and controlled bass makes these headphones simply a best buy in this price range. They have no competition at all in this price category - period.
Whatsoever, many models that cost twice as much, or even more were not able to outperformed them.
And when it comes to bass, here you get clear, beautifully precise and strong Muhamed Ali punches, compare to raw, brutal, wild and uncontrolled Mike Tyson type on the most of the models I've tried.

Highly, highly recommended.

Pros:
- beautiful, neutral, very well balanced overall sound experience. Bass lovers - they are bassy to me, but to some folks they are flat - can be run with ease on mobile devices on pretty high volume due to good impedance (35 ohms)
- top quality build, very durable
- outstanding comfort, they were comfortable over extended period of use (5 + hours)
- supplied cables seems impossible to break
- circumaural design contours around the ears for excellent sound isolation in loud environments (and they do pretty well here)
- collapsible for space-saving portability
- look nice, very nice

Cons:
- they are collapsible and even comes with a travel pouch, but they are quite big headphones. Not travel friendly.
- unfortunately, they do not come with 1.2 m cable like a big brother m50x, so you have to wear 3 meters one with you. Lame, but lower price must be justified somehow.

First of all, I own a pair of white Audio-Technica ATH-M50 and I wanted to replace them because they were a bit old and my cat scratched them (Not badly though, but it was time for a new pair). Anyways, I was about to order the M50x, but in the process saw the M40x, read about them (Specifications and reviews as well) and decided to get them this past friday and got them a couple of hours ago. By the way, big props to Amazon for shipping them internationally, Perú, over the weekend.

Before the review itself, I'd like to add a PROS & CONS in case you don't want to read the whole thing:

+ New design, a bit more minimalist. Matte finish.
+ Little smaller than the M50/M50x.
+ Practically the same sound as the M50/M50x.
+ 40g lighter than the M50/M50x but feels a lot lighter when wearing them.
+ Softer cushions, making them quite comfortable.
+ Cheaper than both the M50 ($20-30 difference) and the M50x ($70 difference).

- The only thing someone might consider as a con would be that the M40x's earcups only rotate 90° instead of 180° like the M50/M50x. Personally, it's irrelevant for me.

Onto the review and wall of text:

As far as design goes, they're pretty much like the old M50s, with a couple of minor tweaks, for example on the earcups you'll just find the A-T logo, instead of the model name and brand. I guess the main differences would be that the M40x is slighty smaller and its earcups won't rotate 180°, but only 90° and for me, that's no problem. Overall, I'd say they look much better than the rest A-T models.

The sound is unbelievable, the difference between them and the M50s is almost unnoticeable because they nearly have the same specifications. I also recorded myself when hearing music in various distances because I saw one review stating that the sound leaked badly, but that wasn't the case. I could only hear my music really low when I put the recorder literally next to the headphones.

Now the game changer for me, feel and weight. According to Audio-Technica's webpage, the M40x weight 240g against the M50's 285g; it doesn't seem like a big difference on paper, but these really feel A LOT lighter making them much more comfortable than the latter by miles. Additionally, the ear cushions appear to be much softer than the previous models, which makes them again much more comfortable, especially after prolongued use.

These are also cheaper than the M50/M50x, duh, so this will get you the best bang for your buck definitely.

Bottom line, if you're looking for a couple of balanced high quality headphones that won't drain your whole paycheck, do yourself a favor and order the Audio-Technica M40x. You won't regret it!