This is an interesting combination of Japanese culture, first you have the weird, a melon flavoured Kit Kat. Secondly you have the importance of regional specialties in Japan, the fact that the melon is from a particular part of Japan, Hokkaido. In Japan local (not just Japan but particular regions of Japan) products are very important to the culture, there are even festivals in the area to celebrate local goods. I can't say that I've ever eaten a Hokkaido melon in my life, but I imagine that they taste OK, I also imagine that there's some kind of festival every year celebrating these melons. Furthermore, I imagine that this Kit Kat came out in Japan during the melon season in Hokkaido, and might have even been available at the festival. I could be wrong about all of this, but it fits the culture of Japan.
The bar itself is actually pretty good for a melon flavoured chocolate bar. I should probably mention that I'm not a big fan of melon, I don't hate it, but I likely would never seek it out either. The thing that makes this melon Kit Kat work is the cream, which might be some kind of mascarpone cheese flavour. It really smooths out the melon flavour and makes that flavour work in the context of a Kit Kat bar a lot better.
I can't say that this is my favorite variety of Kit Kat, but I'm pretty surprised at how much I did enjoy it. If my story up top is accurate, then it's also a pretty creative flavour, and suits the Japanese theme very well.