Preferences once again will be the basis for the answers you receive. The KK will keep the string from the sides of the yoyo, allowing it to spin without rubbing the response. There are some people that believe that the concave shape causes the string to layer, in such a fashion, that it makes multiple layer string tricks more difficult. The kk is a good bearing and should last a very long time.
I have kk’s and I have standard bearings. I just play both and don’t worry about it. They all work for me but I will say that there is nothing as satisfying as trying bearings, strings, and yoyos for yourself. I like many things that others don’t.
It’s good to get others opinions but don’t believe everything you hear as preference is king in the world of yoyo. I always look for balanced reviews, ideas, and suggestions.
Try the kk for yourself and tell us what you think.