I am a major proponent of all kinds of Japanese food, from the highest of haute cuisine from Kyoto ryotei to the humble fare available at a konbini (convenience store). Over the years, I've tried to introduce you all to all kinds of Japanese flavors on these pages, from the traditional to the new.
One great thing about Japan for food lovers that you can get interesting and tasty things to eat at all price levels. For instance, here in France you can of course get terrific food at the high end, and pretty good food in the mid-end. Some staples, like bread, are usually terrific. But mass-produced supermarket-level stuff tends to get very iffy. Japan has its share of supermarkets and convenience stores and icky mass-produced food too of course, but you can still get a lot of great things at the lower end of the price spectrum.
Japanese candy tends to taste really good, at least to me. Everytime I'm back in Japan, I stock up on my favorites and try some new things. My secret forbidden stash of candy is about 90% Japanese*. The Japanese market tends to be quite adventurous when it comes to rather unusual flavors, so you get things like rose-flavored gum and hard candy that tastes like beef, not to mention cherry-blossom flavored ice cream (in the spring) or sweet potato flavored puddings. All a lot of fun.
Which brings me to the Japanese candy product with unusual flavors that gets the most attention overseas: flavored Kit Kats. I suppose they get so much attention mainly because they're an international brand (originally produced by Rowntree's in the UK, now owned by international confectionery behemoth Nestlé). Business publications in particular seem to be fascinated by them; when NPR did a story about them, it was by focusing on the business and marketing aspects  (though the accompanying story has the staff sampling various Kit Kat flavors). They're one of the things that a lot of people interested in Things Japanese (especially the anime and manga set) seem to want to try.
If you can't make it to Japan, you can buy flavored Kit Kats from various mailorder places that sell Japanese products - at a premium of course. But, are they worth it?
Well, no. Flavored Kit Kats are really all about the novelty value, and the packaging, and the strange urge that takes over some people to Collect Stuff.
Let's look at one of the more fun flavored Kit Kats that came out last year. The box is shaped like Fuji-san (Mount Fuji), the most iconic mountain (actually an active volcano) in Japan. This was a special limited edition that was only available for sale in the areas around Mt. Fuji, namely Hakone. If I recall correctly, it costs a whopping 800 yen. But it was so cute I couldn't resist.
The packaging proclaimed that the Kit Kats within were Blueberry Cheesecake flavored. Now, having tried other flavored Kit Kats in the past I was quite skeptical. Still...who could resist that packaging? Inside the volcano, I mean, box, were 9 mini-sized Kit Kats.
Here's how most flavored Kit Kats look. The base for these flavored offerings is usually white chocolate, since it's more neutral than milk or dark chocolate. The white chocolate is sometimes dyed to match the purported flavor (green for matcha tea, yellow for banana, etc.) but in this case they didn't dye the Blueberry Cheesecake Kit Kits blue. Phew.
So...how do they taste? To be frank, disgusting. And that's really not surprising, if you think about it. Flavored Kit Kats are based on cheap, nasty white chocolate, with cheap, nasty artificial flavors and colorings**. With the constant pressure (I presume) to produce new flavors, the quality control may not be quite there. Plus, most of these flavored Kit Kats are limited-editions, so if people are particularly grossed out by a particular flavor no problem - it's just not made any more.
There's a whole market in Japan for something called shokugan, which means "food toys". Shokugan aren't edible toys, or toys shaped like food (necessarily) - it means the extra thing that you get with food. The 'surprise' in a box of Crackerjacks or a box of kids' cereal is a shokugan. In some cases, the surprise itself has become so major that the candy is an afterthought. The Re-ment company , for example, makes very detailed shokugan toys and other things, that come with a nominal piece of candy or gum. (Previous Re-Ment .) Clearly the draw is not the edible part, but the shokugan.
In a way, flavored Kit Kats are bordering on shokugan - they're sheer novelty items. If you expect them to taste good, be forewarned that they emphatically they don't. Of course, they do make great gag gifts for the folks back home, especially if the are not food people per se.
[* I especially stock up on yummy anti-sore throat candy in Japan, which comes in flavors like yuzu and umeboshi with shiso. However, my favorite sore-throat candy is Ricola's Elderflower (Holunderblüten in German; fleurs de sureau in French) flavor, which also has the advantage of being sugar-free.]
[** I'm not a total anti-white chocolate person. In the summer months, several chocolate makers in Switzerland - including Nestlé - make delicious fruit or spice flavored white chocolate bars. But then the chocolate they're using, and the flavorings, are on another level, though the bars themselves are not expensive.]