I had the 615K on the NC the first year I ran it. The tire was brand new at that time, having just received the "K" version over the original 615. I thought they were one of the grippiest tires I'd run to that point, and still think they have better max grip than anything else I've run. They did get greasy quickly once hot, and they heat-cycled out with 50% tread remaining, but I was also new to the car and probably overdrove them a fair bit. Also, ran them full-tread, which I wouldn't do again. Basically, they seem to be a better version of the previous 2 Azenis models, with the same general issues that the 215 and 615 had - poor heat tolerance (compared to Star Spec and RS3) and early heat-cycle death. If they had been cheaper than the competition or superior in some other way, I would have purchased them again. I went to RS3 after those and slowed down a little, but ultimately learned the tricks to the RS3 and favored it quite a bit more, especially in the value, heat and usable lifespan departments.
I heard that this 615K might have had another compound revision, but I cannot verify that. Falken does give away tires through the Discount Tire program! (how I won mine)