I recently did a review of this on Head-fi, and since I really put thought into that one, I will just copy it here. But here is a TL;DR version of it:

If you're looking for a very affordable on-ear headphone and care more about the sound quality than looks, this is the one for you. Best bang-for-buck headphone I found so far, and wholeheartedly agreed in the largest audiophile forum ever, Head-Fi.

Sound quality:
Tight, clean and deep bass, relatively for its price, (coming from a non basshaed) that doesn't intrude on the vocals and the mids, and sufficient highs. The mids are a little bit recessed, but nothing too severe and vocals can still be heard even if its a bassy song. The highs can be a little bit piercing when in a loud environment as the bass is drowned by noise.

Reasons for the score: Sound quality is relative to its price bracket. For around the $20-$50, it's quite a struggle to find this kind of quality, unless of course compared to discounted headphones from higher price brackets.

Design and Comfort:
I write this together as they go hand in hand.

Knocked down on the design a bit because as a portable, it kind of fail, as they are of open-back design. Cannot give it a 8 because of that. Sound leaks in and out of the headphones. While still decent in semi loud environment (walking around, in a bus, on a newer designed electric train) it still is drowned out by loud noises and highly unsuitable for quite places (such as libraries, airplanes, etc.).

But I can't give it a 6 though as the ComfortZone mechanism is brilliantly designed to relief the pressure from your ear to your temples. Defaulting on "Firm", there is also a middle ground setting and a "light" setting where the pressure goes fully to your temple. Mind you the pressure is not too clampy to begin with, but this is a great way to transfer pressure from the ears for prolong use. In my opinion, this really deserves the 8 score for comfort, even if it's an on-ear headphone.

And apparently my attempt to copy my impression from Head-Fi exceeds the text limit.

If anyone is interested in following my impression of it, here is the link.
http://www.head-fi.org/products/koss-portapro-headphones-semi-open/reviews/6271

Conclusion:
Great affordable portable headphone. I recommend doing the "quarter mod" (it's in the link) if you are interested in getting a more balanced, yet still fun, headphone.