There's always one.  Every good thing has a dissenter, and that's me.  The ATH-M50 cannot handle songs with lots of different lower frequencies going on at once because of the artificial bass extension.  For example, Nine Inch Nails' "Heresy" is a very challenging song to play on headphones; it sounds like crap on my car stereo because the subwoofer is confused on which notes to play, so too low of frequencies overpower what should be playing louder.  The M50s do the same on the FLAC version of it.  The impacts of the kick-drum are far quieter than the bass in the verses.  It's incredibly annoying.  The bass is so sloppy and uncontrolled.  Another good example, Smashing Pumpkins' "Thirty-Three", in which there is a subtle background bass that tends to get incredibly emphasized to the point it's unlistenable on car subwoofers/headphones with poor bass response.  The ATH-M50 do this.  Luckily, they have really nice mids and respectable highs.  Yet, it's shocking they bill these as monitors, they're horribly unbalanced towards the lower-end.  These headphones main competitor, the HD280Pro, are superior in every way, except for the clamping force.  Not a producer I can think of uses these, either.  NOT monitors.
 
The build quality is a joke.  I went through three earpads in a year before the right speaker stopped working and the connector to the iPod got frayed.  They seem big and durable, but I've had lesser headphones be more durable.  I thought they were comfortable.  But the praise on here is sickening and misguided.  Do y'all get paid by Audio-Technica?  And at their new price, they are an even worse deal.