I used to have a pair of Koss Portapros before I got my M50s so I was interested in having better headphones than my usual phone earbuds. After reading the reviews here, I managed to get a used pair of M50s for £85 which seemed like a good bargain to me. After around 2 weeks of stretching out the headband and getting used to the flatter sound signature of these as compared to my earlier (bass-heavy) Portapros, I have to say that my lasting impression is one of awe at how these manage to tick all my boxes. 
 
They're really portable, fold up easily into the bag and can be carried around quite simply in my backpack. I usually listen to music through my Nexus 4 which commuting and my laptop while at work. Both can drive the M50s very easily. Moreover, my music now sounds great!! I love the instrument separation, the decently strong and tight bass and the way electronic music seems to suit these so well. Not being an audiophile of any kind, I'll refrain from commenting on the finer details, save to say that these have a very pleasing sound signature and can make any kind of music very enjoyable (best with electronica though) They're not that heavy which makes them very comfortable and the cord isn't great but you get used to it. 
 
Overall I'm thoroughly pleased with them, and would recommend them in a heartbeat.