Cons:Extremely uneven frequency response. Sounds bad in almost all situations.
A couple of years ago there were not all that many in-ear headphones to chose from and Koss Plugs were the only cheap ones. At that point there might have been some kind of justification in buying them. Things have change and now there are numerous in-ear headphones of varying quality available. Sadly the Koss Plugs are among the worst of the lot. These are the reasons to avoid them:
The bass response is massively over-exaggerated. This is worsened by the fact that the bass is also very inexact, i.e. everything in the low frequency span sounds pretty much the same. A drum is a drum, is a drum. The treble is OK while the mid range is very muffled.
The total effect of the uneven response over the frequency range is that music sounds "strange" no matter how much you tweak your player. Vocals sound especially bad. Bass heavy music is completely destroyed.
Although these headphones cost very little I can not say that they are good value for money.