Tejal Rao writes in The New York Times Magazine:
The Kit Kat, in Japan, pushes at every limit of its form: It is multicolored and multiflavored and sometimes as hard to find as a golden ticket in your foil wrapper. Flavors change constantly, with many appearing as limited-edition runs. They can be esoteric and so carefully tailored for a Japanese audience as to seem untranslatable to a global mass market, but the bars have fans all over the world. Kit Kat fixers buy up boxes and carry them back to devotees in the United States and Europe. All this helps the Kit Kat maintain a singular, cultlike status.
This article published last year, but I find it fascinating.