I bought these headphones to replace the earbuds that came with my iriver h10 (sennheiser mx400's).
The most obvious difference between the two is that the sennheisers, and all other stock phones that come with mp3 players, are ear buds, where as the koss ksc75's go over the ear. So, to compare
Sounds.
For the price you will struggle to beat the quality. In fact you will struggle to beat it if you pay 3 times as much. Excellent for anyone other than a true audiophile, and far and away better than the ear buds.
Looks.
I like how they look anywhere but when I am wearing them. There is no way to get around the fact that you look like a bit of prat with them on. Personaly I think they make you look a bit like shrek from behind.
Comfort.
Comfier than most bluetooth headsets, but not as comfy as a pair of glasses. They attach behind the ear just like a pair of glasses, but obviously weight a little more and so put slightly more pressure on your ear. Not enough to be uncomfortable, but you are aware you have them on. I have worn them for in excess of 3 hours without a break without feeling distressed.
Isolation.
Non existent. You can still hear everything going on around you, but it is dimmed down. Turn up the volume and everything else disappears. These headphones sit on your ear, they dont enclose it.
This works the other way also, everyone can hear what you are playing. Dont expect to play loud music with these headphones whilst on a plane and not get stared at by other passengers.
Misc.
Chord length is about perfect, plenty long enough to have the player in your jeans pocket, but not have the chord trailing on the floor.
Summary.
An excellent candidate for your money when replacing the stock earbuds that come with your mp3 player.