I have the ATH-M50, and here are some genuine observations:
Edited by Boullion - 9/1/12 at 6:50pm
1. They do offer a decent amount of passive isolation. Hardly any noise leaks out and not too much goes in.
However, I took them on a plane and they didn't fare very well in isolating the noise. I had to crank the volume up considerably (~50% on my iPod Touch).
If you're looking for airplane-level isolation, I would recommend an IEM (in-ear monitor) or the Bose QC3/QC15, which has excellent active noise cancellation.
2. They sound fantastic to me. People say they're bass-heavy, but to me they sound relatively balanced (but yes, with a focus on bass). However, personally I'm not sure if it justifies the $128 I spent on them. They are suitable for a very wide range of music genres, which is nice.
3. I find them to be severely uncomfortable. This is why I don't like the ATH-M50. I thought it came with bad pads, so I got new ones. Well, they're still pretty damned uncomfortable. Not to mention the amount of padding on the headband is abysmal. If it weren't for the comfort issues, I would enjoy these headphones. And so, I would recommend looking for other headphones (mentioned later).
4. They are easily driven with a portable music player. However, the ATH-M50S has a very long cable and the ATH-M50 has a bulky coiled cable. If you're thinking of using this as a portable headphone, I would think twice and get something like the V-MODA Crossfade M-80 or the Sennheiser HD 25-1 II / Amperior instead.
I would recommend looking at other choices before diving straight for the M50 (advice I would have told my past self).
By no means am I an expert, but here's some other headphones in a similar price range to consider:
-M-Audio Q40 (bass-heavy)
...and others that I can't remember. Recently I've been looking at portable headphones, sorry
I know you don't want these recommendations, but honestly I would give other headphones a shot.