I have the AT M50's and use them as my main headphones. They are decent for the money. Here is a review of several headphones that you might want to read:
http://www.soundonsound.com/sos/jan10/articles/studioheadphones.htm I am considering getting the AKG K240's as a backup to my M50's, not for tracking, but for mixing, not in place of studio monitors, but just as another point of reference, a different point of reference.
I also have the bigger brother of the Yamaha HS50m--I have the HS80M. I really like them. With any studio or headphone remember that you must use them a lot to get to know their frequency bumps and valleys, so you can make the proper addjustments to your mix.
You might also consider a VRM for headphones as it givies several different modeled rooms and speakers so you can hear what your mix would sound like in different settings on different speakers--all with your studio headphones. This would give you a different perspective on your mix.
Finally, with any studio monitors it is important to have quality room correction. You need to know your room, it's frequency bump and how to correct for it. I use ARC 2.1 from IKMultimedia. It is wonderful for correcting the bass bump that small bedroom or project studios often add to the sound in the room. By correcting this, it help you get a mix that translates beyound your room to other settings, other rooms, other cars..... All this still is contingent on the mixer still listening and making careful corrections to the mix. Learn your room, learn your monitors, learn your headphones so you make informed corrections.
Do look at that soundonsound review of headphones. It might give you some clues about different options.