First off, Id like to say that Ive been a long time snooper here, but I felt its time to give back.. so here goes:
I just received my M50's after doing some extensive research... I was looking for some good cans for under 150, primarily for Studio Monitoring use. I produce music, mostly soft rock, electro-rock, and pop-rock. Ive used the Sony 7506's, Senn 280Pros, Sony V700DJ's and now the M50's, and here is my feedback:
Sony 7506
These sound great, and they have a lot of detail. But as many have stated, they are definitely colored. They sound great, but for production use, they are inaccurate. Tracking/recording use is fine, but the mixing is where the weaknesses show up. I end up mixing my tracks a certain way that sound different in every other stereo set up after-the-fact. The bass and treble is emphasized, slightly like turning on the "powerful mode" EQ setting on some home stereos. But on the flip side, these are great cans for just enjoying music through your ipod/computer/etc. These are not as comfortable as the Senn280 or the M50's, as the cans sit on your ear. Dont get me wrong.. these are comfortable, its just that the other two are WAY more comfortable.
Senn280pro
These are great cans. Very neutral and flat... which makes them way more accurate. Very detailed, but not overemphasized. The low end, however, is lacking. I dont get some of the bass resonance that makes kick drums sound beefy, nor the the rumble of 808's. For standard listening, this can be a turn off, as songs seem to lose a bit of life through these headphones. Definitely more for critical listening rather than enjoyment listening.
Sony V700-DJ
These are designed for DJ use, and honestly, they are all these are good for. Great pounding bass, emphasized highs. Almost crap in between. Great SPL, so in a loud club environment, where you need to hear the other song you are about to mix in, its great because it makes beatmatching that much easier. But forget about any quality listening. But if you are a bass monkey, you might like these. These are not as comfortable as the Senns or the AT's.
M50-
I just received these, so they are pretty much factory fresh. I ran them for a few hours to burn them in, but I still have a ways to go. But so far, they have more lower end than the 280pros.. the same song that seemed weak on the drums now sounds more beefy... kind of like the drummer just woke up and realized hes in the middle of a performance. But it wasnt a HUGE difference... 808s are more present, however. The highs are a bit brighter, but not sibilant. The mid bass does sound a tad muddy, but Ive read that after burn-in, they get clearer. Mids sound a hair recessed, but again, hopefully a complete burn in will resolve this. I suspect these are better to rap than the 280's, but I still wouldnt say these are good for using with portable music players.
Overall, the Sen 280pros are the most neutral, the M50 is slightly bass-emphasized over the 280's. But if you are looking in this range and you are looking for rap music, I would suspect that the Sony may serve you better. Typically, rap songs have emphasized highs, with prominent hi-hat/other tweeter elements, with good beefy bass lines and 808's with heart pounding kicks. The Sony 7506's sound great here as they seem focused on these. If you really want SPL bass from headphones, Sony V700dj's all the way. With these cranked up, the 808's practically make these cans vibrate off your head.
Hope this helps.
Oh by the way, if you REALLY want affordable portable earpieces and you dont mind the In-ears, the SONY MDR-EX90LP's are actually pretty awesome... I personally liked them a bit more than the ER6i's and other in ears that cost 3x more! ****BE CAREFUL OF FAKES******THERE ARE LOTS OF FAKE EX90LP's OUT THERE*****