When I was in Japan I think I came back with 7 different flavours of KitKat. And they all (but one or so) tasted really good. It's too bad they aren't available elsewhere. Another type of Candy where the Japanese get creative with flavours are Pocky sticks. There's always a flavour you didn't try yet.
I can get matcha flavored KitKats at a local store here in the US.
This is just my personal observation with no data but AFAICT Kit Kat is *mostly" for tourists here in Japan. If there's an obsession it seems to me it's tourists buying them. I could be wrong but I've never personally seen Japanese people buying them (of course they must, just saying that I haven't noticed). I just walked over to 3 convenience stores that have a large candy selections. The 7/11 had no Kit Kats out of…
I saw a very popular seasonal KitKat stand within Seibu department store when I was visiting Japan, and I didn't notice any other tourist-looking people around. Is it possible that it's just not popular in the convenience store market, but is more popular in competition with all of the higher-end sweets people were constantly giving me, talking about, and eating while I was there?