Pros: Inexpensive, fantastic soundstage/imaging, incredible instrument separation, very detailed, textured bass, crisp highs, energetic and engaging sound
Cons: Clips aren't the most comfortable, aren't sexy looking
Taking price into account above all else, these are the best headphones I have ever owned. Obviously my HD650s and DT880s sound better, but they also cost 15-25x as much. I've probably put ~150 hours on them. I did do the kramer mod (involves drilling a few holes in the driver cover to allow more sound through) but they sound great even without the mod. I also bought a Parts-Express headphone from Amazon, took the garbage drivers off them and snapped the Koss drivers on. I find them much more comfortable this way. As a plus you can bend the headband however you like to suit your comfort level. Some people do the "quarter mod" where you put a quarter over the earpads (after taking them off the drivers) and cut out a circle to allow more sound through as well. I chose not to do this because I love their sound with the kramer and headband mod.
I listen mainly to electronic music, so people who listen to more jazz or rock might not like these as much as I do. But for trance, dance, dubstep, these are simply amazing. My DT880s show me new things in my music that I've never heard before. It's one of the things I love most about them. Now this isn't me blowing smoke... but there have been times I heard something on the Koss that I've never heard before. This includes faint vocal tracks in the background, synths, and little things here and there.
These are my work headphones, because they allow me to hear people talking and not completely isolate me from my environment. Some people say they leak too much sound to use in public, but I use them in my cube and no one has ever complained about hearing my music. I don't crank them up but I certainly don't listen quietly. I use them straight from my phone (Nexus 4) and they sound great. I brought my Fiio E17 to work once to try them amped and I just felt like they lost some of their magic. Don't ask me why, or how, I don't know. All I know is that they didn't have as much sparkle, which is something I love about them.
I'm no expert in describing audio, but I'll do my best. They have a great attack and decay on the notes, which to me translates to good PRaT (pace, rhythm and timing). The space between notes is very discernible and they just get me groovin'! The imaging is fantastic as well. I can clearly hear music going from side to side, and even in front of and behind me to a degree. The bass is punchy and extended. It also has a great texture to it. It's not muddy or bloated at all. They're also very airy and have a nice open sound to them. The treble is sparkly but unfatiguing (in my opinion). Some people find them a bit too treble-forward, but I have experienced almost no sibilance from them. Mids and vocals are good, nothing amazing but they're not recessed. Female vocals are quite nice on them, with male vocals almost as good.
I honestly think these are worth $75. I might be slightly more critical of them at that price, but these are so cheap, I have literally nothing to complain about. I don't plan on recabling them because I love them just the way they are. I bought 3 pairs over the holidays to give to friends. I only ended up giving away 1 pair. These are just... so good. I'm more satisfied with these than I am with my HD650s.
What Koss has done here is simply magical. I would never have dreamed something so good would be so cheap. I honestly could not love these more. They are my favorite headphones out of all that I have ever owned.
Thank you, Koss. THANK YOU!!!