I own 4 pairs of ath-m40's. I did a lot of research on good budget tracking cans for my studio. I read a lot of good things about the m40's and found a used pair. I was so impressed I bought three more new. At $52 new these are a great deal!
These are rugged cans that are totally field serviceable. The plastic is very hard and high quality compared to other cans in its price range. The 1/4 chord works great in the studio but is not good for laptops or mp3 use. I replaced one pair with a 1/8 jack for daily use.
The sound quality is very nice and flat. Users of normal headphones might be surprised by the flatness as most phones boost the high and lows. I personally love the detail and flatness. The phones are far more detailed than anything in its price range. Make sure you break them fully to obtain a natural sound.
One flaw is the pads on these phones wear out really fast. With normal daily use the pads start to wear out in less than six months. The pads are really cheap but can add up fast if you are using them a lot. I did a lot of research and took a chance on putting some shure shr840 pads on a pair. To my delight the fit and improved the sound isolation quite a bit. This is good news because the 440 pads and 940 pads may also fit.
These phones are comfortable, rugged, and sound great. The 11' long cable is nice and long for studio use. The isolation is also fairly good and can be improved with the shure shr-840 pads.
I demo'd the ATH-M50, ATH-M40fs, and ATH-M35 side-by-side at the Audio Technica booth at AES NYC 2013 in the Javits center a couple of weeks ago. It was in a noisy exhibit hall, but all three headphones were plugged into the same source. All three sounded quite fantastic. Low end extension is excellent without boominess or muddiness. The M35s were definitely a little bit more muffled and less clear than the M50 or M40fs, so I focused on comparing the M40fs and M50 to each other. Both of those two models were wonderfully clear and detailed in the mids and high end.
I actually found the M40fs to give more punch to the snare than the M50s did in the jazz/fusion track that was playing. Otherwise, I found these two to be virtually identical and had to go back and forth between the two several times before I even picked up that difference between the M40fs and M50. Perhaps some of the similarity of the two had to do with the particular source or the noisy tradeshow floor environment.
But bottom line, I immediately purchased the ATH-M40fs, and they sound terrific.