I just picked these up for my girlfriend as her christmas present. They also came with the IEM pulse KEB40 as well for free :)
I own Sony mdr v6, Sennheiser 595, and Denon A2000. I've got a Superpro 707 DAC and Total Airhead Amp.
Obviously my daily listening is with cans much better than these, however, first impressions are good! They need burned in, but so far they seem pretty good, though they clamp a little too tight for me, though it's not a fair comparison with the 595s or A2000s as those are in another class. For 50 dollars, they seem pretty decent. They do feel kind of cheap to me but what can you expect for $50. I like that Koss has a lifetime warranty, which I've used on another set of headphones before and they replaced them with zero problems.
The bass on these suckers is very prominent in a good way. The mids are quite a bit muffled right now...but we'll see how they sound after burning them in. Right now I dialed down the midbass hump and turned up the highs just a tad and they sound more pleasant to me, but that's personal preference.