I was pretty particular when it came to dropping $100+ on a pair of headphones, especially when I already had a nice pair of Sony MDR-7506's at my disposal, but boy was I glad I pulled the trigger on this purchase. After checking review-after-review (YouTube, forums, etc.), I can most-definitely proclaim that these headphones are PHENOMENAL. Easily the best pair of headphones I've ever used.
Volume: First, the ATH-M50's require an exceptionally low amount of power, so it won't take a lot to get these babies blasting. I pretty much always do my listening straight from my laptop, and on occasion through my HD display with my PS3, but you can even crank em' with a modest mp3 player. You can get these headphones up to incredible levels, and it's as though nothing ever gets distorted (like bass, etc.)
Comfort: The M50's padding feels great. Audio Technica leaned towards the generous side with the padding, so from the start these headphones are already WAY more comfortable than my Sony 7506's. I have a rather large noggin, but these fit snuggly and never seem to get too hot, plus, again, the padding feels terrific. You can seriously listen to these for hours without having to pop them off for a breather.
Sound: The low, mid, and high frequency response is gorgeous and completely flat (flat meaning that you're hearing the music without the headphones adding in addition bass, treble, etc. Everything you hear is the way it was meant to be heard. You can jack up the bass, treble etc. with an equalizer later, if you want to, but I hardly ever do.) This is perhaps one of the most superbly balanced pair of headphones you'll ever use.
These headphones are the reason why you'll go out of your way to find lossless WAV and FLAC audio files online, because every pluck of the guitar string, every hum of the violin, and every boom of the bass sounds just that much better here. For you bass heads, the bass here is very clean, without ever getting distorted. I think I could have considered myself a bass head for awhile, but these headphone got me to realize that there are also highs and mids, not just lows, and that you often don't hear it all with other headphones. Either way, these puppies still deliver in the bass area, and if you need more just download an equalizer (I use VLC for my music, equalizer, etc.)
Here's a quick comparison to some of the other "leaders" in this genre. I've personally tried these:
--Sony MDR-7506's - A great pair of headphones. My first pair of high quality headphones. While these are the industry standard and do sound terrific (you'll oftentimes see them in music videos, and for good reason), and I'd recommend them to anybody getting into the audio scene, when I cranked these babies, the highs were a bit too sharp sometimes and weren't pleasant to the ears, and the lows tended to get a bit "boomy" and distorted at very high volumes (otherwise they're nice). Plus, comfort-wise, my ears would often hurt after extended listening sessions, and you'd often have to pull them off to let your ears breath/cool down.
--beats by Dre - These headphones rely far too much on exaggerated heavy bass and sharp highs, which oftentimes drowns out the mids completely. The ATH-M50's are FAR more balanced and detailed. You'll most definitely be able to hear the difference. Not to mention, value-wise, beats by Dre are INCREDIBLY overpriced. To each their own, but these definitely push too hard in certain areas, and not enough in others. These are definitely a few steps below the ATH-M50's.
And while music is where these headphones absolutely shine, I have used them for gaming as well. I did play through Killzone 3 in its entirety with these on, and it was an awesome experience. If you don't need to have a mic attached for online chat, these will do quite nicely if you need to keep things quiet around you, or you just want to make things more immersive.
Overall: BUY THESE HEADPHONES. I went back-and-forth on Amazon for MONTHS wondering whether or not I should grab these, because let's face it, for some folks this is a pretty important investment (you don't want to throw $100+ away), and there aren't too many stores around that will let you take headphones for a test drive (at least not many that I know of), so you have to go strictly off of online reviews and videos. I was satisfied with my Sony 7506's, and didn't think I really needed to go any higher, but boy was I wrong. Everything about these headphones is phenomenal, and far superior to most or all you've probably heard. From the AMAZING sound quality to the excellent build quality (did I mention these are super solid/sturdy without being all that heavy?), I can essentially guarantee that you won't be disappointed if you order a pair. If you want to hear audio the way it was MEANT to be heard, grab a pair of these and get blown away.
P.S. If you're considering buying these for someone else, I bought a second pair for my parents who are big on old-school dance music (60's, 70's 80's, 90's, funk, disco, etc.), and they have seriously put the M50's through its paces, and the sound has only gotten better. As other reviewers have noted, the sound actually seems to get better the more you use them, so if you grab a pair, don't be afraid to turn it up to 11! If you crank these headphones, they'll thank you for it, and ask for more!
I feel like I could go on-and-on with this review, so I'll cut it off here, for your eyes' sake. Good luck with your decision, and in the end I hope your ears are happy (mine are!)