I completely agree, the k90 and in fact, pretty much all of the tour 90's in my opinion are great racquets. Not super easy to play with but if I'm ever playing against weaker players, I often use it as playing with it is really fun. I pull out my St Vincents sometimes as well under similar circumstances.
Against guys that rip it, I find myself late and particularly later in the match, find myself tiring as it is, particularly in the retail stock form, a little demanding for me. That's why I'm particularly enjoying this pro stock one, more head light than the retail kfactor and nowhere near as demanding. If I leaded it up like Fed's, different story I'd imagine. If I was a true Fed wannabe, I'd have done it. I also tried his string setup and struggled for control after the first hit, I prefer these racquets strung at early 50's in full poly, really great spin for putting away short balls, love how easy it is to move the small head.
My main observation out of all of this is that I think more Fed and Tour 90 lovers out there should get hold of Asian Tour 90's, put silicone in the handles and enjoy!