I understand the hype for the M50, I think it's a good phone, and the perfect one to recommend to potential Beats customers; it has enough bass impact and treble pizzazz to impress someone who is new to nicer headphones. It's also built like a tank and is collapsible, which kicks ass.
 
However to me, the places it falls short for the price IMO are its mids and presentation. The Senn HD439, which is around the same MSRP but easily found for $50 refurbed on Amazon or eBay (DAHMART refurbs are good, I've had 2), sounds nearly open by comparison, and has a level of midrange detail and fullness of sound that I think easily outshines the M50, which is dry and a bit hollow in the mids. It also has a replaceable cable, which is huge for me personally.
 
Then again, the HD439 is built of cheaper materials, doesn't isolate as well, and isn't collapsible. I think they both occupy a similar place in the market, though, and I wish the HD439 got as much mention as the M50s, because they are a great set of cans and can be had for a huge bargain right now. They cater to similar price points and uses, but different listening styles, and, IMO, should be mentioned in the same breath to people looking for good semi-portable cans in the $100 area.