I have owned two pairs of these: the second pair were a replacement for the first, which stopped working after some two months.
The second pair lasted three months - then yesterday they stopped working.
The malfunction was the same as the first pair: audio in one ear, but if you rotate and fiddle with the jack, you might get audio in the other ear, too.
The sad thing is, I went with Koss because they were meant to be a reputable earphone maker, and might be better than Sennheiser, or Sony, or Panasonic. They've turned out to be worse - which is a shame, because they have good sound quality...
Avoid.