Pros: Unbelievable instrument separation and "energy"; balanced sound
Cons: For the price, nothing; in absolute terms, things are a bit "hollow" and not full-sounding, could extend better on both ends, and clips are a pain
These headphones have it all for the price. I've never heard anything like them at this level. They have a clear, balanced sound with all frequency ranges represented fairly equally, with mids and highs ever so slightly forward compared to bass in "clip-on configuration". If you mod them with a PortaPro or Parts Express headband, the drivers are pushed closer to your ear and give a bit more bass and mids.
The main thing that stands out with these is a simply silly amount of instrument separation. Everything sounds like it is popping out of a pure-black background -- think of turning the contrast up on your TV or monitor; or more correctly, adjusting the gamma level. This lets you analyze tracks very easily, useful if you're a musician. A downside to this is that they don't sound too "cohesive", and music can sound a little hollow, flat, or broken-up. Don't confuse this in any way with "dull", though, because they are full of energy and never fade into the background, while still not being harsh.
A fairly unique sound in absolute terms, and a steal at the price. The only downside is the comfort of the clips; I could never get them to feel great, but they were at least functional. Good work Koss.