Here goes my short review on the Audio Technica ATH-A900X:

Have been burning in the cans for bout 20-25 hrs. So far so good. I have to say, maybe others might have their own opinion but for me, the A900X has one of the widest sound stage ive heard so far for a closed back pair of cans.

Before burn in the drivers produce a more "Boomy" bass as compared to a fully burnt in M50. But after several hours or so, it becomes more pronounced. But for me, the M50 still produces a more punchier bass than the A900X.

The sound produced?
Lets start with the words, Bright and Accurate.
The lows, bass response is great, as these are closed back cans. Dont expect head banging bass, as these cans are not for bass-heads and they dont have that extra PUNCH, can go for BeyerDynamic! Because of this, the mids are more pronounced.
The mids are crystal clear, articulate and warm. Its not fatiguing/tiring to ones ears. One word : Awesome!
The highs are smooth and doesn't hurt your ears.
Tracks used, there are various, but these were some of the prominent ones:
The Four Seasons - Vivaldi @ 610kbps FLAC
Cloud Atlas Sextet - 795kbps FLAC
Swan Lake - Tchaikovsky @ 585kbps FLAC
Moonlight Sonata - Beethoven @ 525kbps FLAC
(the list goes on...)

Games tested:
Battlefield 3
Medal Of Honor Warfighter
Crysis 3
Planetside 2
Metro : Last Light

Tested games with Dolby Headphone Surround 5.1 / 7.1 software via my Asus Xonar Essence STX. 

How does the A900X perform in games? 
For those who never used proper headphones before this would only be saying WOW!
Really great sound stage, its F-ing WIDE. You can hear every detail. Especially in Battlefield 3, when grenades or tank shells explode right next to you, for me, I can even hear the grain of debris/sand as they fly and and hit the ground back. Played through the "Going Hunting" SP mission, I could actually hear the radio chatter of the pilots in action!

Positional Audio?
To put it simply by saying, you can hear everything. It all depends the skill of the player on what to do next.
Comfort wise, its the A900X. But this model will nt really appeal to those whom have smaller heads. Cuz the headband might slip off. But all in all, After I put them on, I forget that its there. While the M50 has a more tighter fit/grip to a person's head.

Drawbacks?
Since these are closed cans, it gets hot after a prolonged session.
Not for the travelling user, the cans are bulky and the cable is like super long
Take note that these cans are also HUGE but lightweight due to its bare bone headband design.

"Gaming" headsets Ive used/tested before? 
Roccat Kave 5.1
Razer Tiamat 7.1
Razer Electra
Corsair Vengeance 1300
Various Armageddon "forgot the model"
(Numerous others...)

Conclusion: None of those listed above can stand nor be compared to the A900X in terms of sheer audio performance and value. In other words, if you think you have money for a Tiamat or I would say "Kiamat", you`d be better off with an M50 or A900X. If someone plays an MP3 track, lets say with 320kbps quality, they will notice instantly how bad MP3s are.
The A900X is a really great ALL-ROUNDER and BALANCED pair of headphones or entry level audiophile cans as they say.

Though my review might be biased, as my setup also uses a Asus Xonar Essence STX, but my stand is still firmly the same as bro @SSJBen, to NEVER go for 5.1/7.1 "gaming/actual" surround unless that person is a mutant or alien.

Cheers & Peace out!