Great sound, great product.

I have used the KSC50 sport clips for a very long time and when I first saw the KSC75s I felt I needed to buy them, mostly due to the fact that my KSC50's had a near severed cable.

Koss continues use of the same excellent driver the KSC35 and KSC50 has in the KSC75, so if you have used a KSC50 or KSC35 in the past you will know what you are getting. Mind I note there are other KSC line products (i.e. KSC55), it is just the KSC35/50/75 all have the same design, where as the KSC55 are an implementation of the KSC50 in a new design with a rear head band.

If you already have a KSC35 or KSC50, there are really no improvements in the sound category. This is not a bad thing, though.
For those unfamiliar with this line of products, the KSC line of Koss products produce some of the clearest, loudest and overall best audio quality I have experienced from a headphone. The bass response is the best I have heard as well. Bass is clear and very enjoyable when listening with these headphones. There is no buzz, no crackling or any of that. The best thing about this line of products, and Koss products in general, is that with more use Koss products improve in sound quality. So if you think the out-of-the-box sound quality is going to be great, just give it a few weeks of use :).

The only thing that really separates the KSC75 from the KSC50 or KSC35 is the design. The KSC75 appears to be more of an improvement of the KSC35 than the KSC50. Comparing them side by side, the KSC75's design mimics the KSC35. The KSC50 is in a league of its own with design. Its a design I really like, by the way, so the KSC75 design was news to me when I bought it.
Both the KSC50 and the KSC75 are still continued products. The KSC35 is discontinued to my knowledge.
Comparing the KSC75 and the KSC35, it would seem the KSC75 is just a newage upgrade to the KSC35. New colors, new rounded edges, new clip coverings, etc. I never used the KSC35 so I can't comment much more.
Comparing the KSC75 to the KSC50, assuming no knowledge of the KSC35, it would seem that the KSC75 is a completely new design.

The KSC50 has a very innovative clip design. It is a rotating clip which is very easy to operate in putting on ones ears. I love it. Koss abandoned this design and returned to the KSC35's clip design. It not a terrible one, but I prefer the KSC50's design, especially after failing to enjoy the experience with the KSC75's clip. I found that it is very hard to adjust the clip in a direction normal (perpendicular) to the headphone, while adjustment tangential to the headphone is quite easy. The KSC50 on the other hand was very easy to adjust in both directions. This is due to the fact that the KSC75's rubber clip is reinforced with rebar (short for reinforced bar, rebar is the term given to the metal bar in reinforced concrete), where as the KSC50's clip is not. You will find that the work necessary to adjust the KSC75 in a normal direction doesn't pay off that much, as it wont return great fitting results. All the fitting will come from tangential adjustment.
However, the KSC50's clips did cause pain to my ears; you will find the KSC75's clips will make you forget you are wearing headphones.
But its a trade off...pain with no adjusting, or no pain and a boatload of adjusting.

The other thing about the KSC50's clip is that it felt very tightly held. Not that I tried to pull the clip off, but I don't think it is possible anyway. On the other hand, the KSC75's clip is VERY easy to snap off. The force required to adjust the clip in a normal direction will, in most cases, pop the clip off the headphone. And from what I can see, if popped off enough times it will wear down on the plastic holding the clip in place. So just be careful.

The cable hasn't been improved. Both the KSC50 and KSC75 sport a very short 3 foot cable...bleh.

One major improvement, though, is the cable itself. The KSC50 sports a wacky cable. From the jack comes a single cable. It is then lead into a splitter of some sort which then splits it off the cable into two cables, one to each headphone. Basically, it is an indication that each of the three cables is lined with its own rubber insulation, i.e. the rubber lining is not continuous. I found out this ended being a problem as the lining split away on one of the cables from the splitter, exposing the wiring inside, and almost causing complete cable severing. I was able to replace the unit though. The KSC75 returns the cable to more traditional means; two cables, molded together, are led from the jack, and then the molding is split to both of the two headphones, with the point of splitting being held by a plastic ball. This shouldn't experience the issue I saw in the KSC50.

One thing that hasn't changed, thankfully, is the price. The KSC50 was $15 and so too is the KSC75.

Overall, at $15 the KSC75 are a steal. They sound AWESOME, have great bass response, are very light, and are a double whammy on those iPod headphones: not only are they better than the iPod headphones, but people won't suspect you have an iPod if you are using these headphones, in other words, you are less likely to have your iPod stolen while using these headphones.

Compared to the KSC35's I think it is a needed upgrade...new colors, new cool look, and probably a better clip. Compared to the KSC50's, its an acceptable upgrade. I really liked those KSC50's clips, darn.