I just got my KSC75s today. Biggest issue I'm having is comfort on the right side. I bent the metal clip a little akward and, despite the fact that it looks just like the left one, I can't get it to feel just right. Granted I have to get used to that band behind my ear as well. I was also expecting the clips to be made of a softer material. It's a plastic softer and less stiff than the KSC35s but I was expecting something supersquishy like silicone.
They sound great, I'm really impressed. As far as burn in, there is a fuzzy kind of buzz on some high frequencies, it should go away in time, I think it already has a little. I'm really shocked at the bass impact these things have, for the size anyway. I use an EQ that is slightly parabolic looking (with the mids being a little recessed) and the treble and bass pushed up a little in the Windows volume control (Audigy 2 here). I've tried without EQ and treble/bass adjustment and...well, everything just sounds like crap. On any headphone I've used...not sure how people go without some kind of tonal control. :/
Anyway, overall I think they at least rival my HD497s. They're a little warmer, less dry sounding. They seem to slurry some of the mids together, but it's hard to tell without a good burn in.