All images are taken from Nestle Japan's web site.
Nestle Japan announced its newest in its vast line of regional KitKats on October 9, 2012. This one is for Tokyo and it is rum raisin. The blurb on their site for this says that it has a sense of sophistication and luxury, and that image is suitable for Tokyo. You know that's the truth because of the elegant snifter of rum that is part of the illustration on the box. I'm guessing that if they put a drunken pirate with a mug of grog on it, they'd believe it was a little less classy. This seems to be replacing the soy sauce Tokyo regional flavor, which has been around for donkey's years. However, I imagine the soy sauce variety will find its way back into the rotation at some point.
Regional Japanese KitKats are the holy grail for many aficionados of bizarre flavors. Part of the reason for this is that they're supposed to be something you only can get by visiting the region that it is introduced in. As I've said before, this is not always so. They are sold in Ameyokocho at Niki no Kashi as well as at Narita airport. The truth is that they are not reliably available anywhere, not even in the regions they're sold at. It's not like you can walk into any convenience store in Tokyo or train station kiosk and find this Tokyo regional variety in Tokyo. You always have to be at a shop that carries specialty or souvenir items. Such places are often at the major train stations of the areas.
Le Lectier (Niigata specialty)
Many of the regional KitKats are sold in boxes with 12 mini bars for 840 yen ($10.52). Just to refresh people's memories about them, they are marketed primarily for people to buy and give to others as a souvenir of an area they are vacationing in or doing business at. They are not really designed for the average snacker to take home and eat on her own. In my opinion, based on my knowledge of Japanese culture and tastes, the strange flavors are not meant to be the sort of thing you crave again and again in the future. They are novelties, much like cactus candy and venison jerky.
Though the regional varieties are supposed to be special, many of them have been issued as regular KitKats as temporary flavors (and you can buy most of them through Amazon Japan). The KitKat zunda (above) was issued as a regular bar after the earthquake on March 11, 2011 and continues to be available in the regional format. Similarly, the blueberry cheesecake KitKat (pictured after the 1st paragraph), was issued in several incarnations while I was in Japan (single finger, regular bar) and continues to be a Koshinetsu regional variety. The apple KitKat is also a regional issue (Shinshu), but was once a short-term flavor for a standard-size bar. My point is that, if you can't get a regional flavor, you don't have to really sweat it because chances are you will have or have had access to it at one point or another as a regular bar.
For those who need fewer souvenir bars, there is also a smaller box available for some flavors in some regions. Those boxes are 350 yen and include 5 mini bars. The one pictured above is hojicha flavor, of which there was (and may still be) a larger box available at one time. The wasabi KitKat that I reviewed came in one of these smaller boxes. However, there is absolutely no difference between the versions as sold in the various incarnations in terms of flavor.
In my experience, some of the regional KitKats also showed up in UFO Catcher games, though it tended to be something which was quite unpredictable. The yubari melon (cantaloupe) version seemed to be especially popular as a prize as I saw it several times over a long span of time. I wonder if it was chosen because it was especially desirable or sold poorly. My guess is that it was the former. This particular variety, I should note, breaks the general pattern of the way the regional KitKats are presented. It has 4 bars for 580 yen ($7.26).
I did not try every regional KitKat, but I tried a fair number of them. Frankly, I was rarely blown away, but I found most interesting and enjoyable enough to finish the box. The very best one and the only one I could say I'd actively want to buy again was the Golden Citrus blend. The rest were mere curiosities, some pleasant and some less so. The citrus blend is currently one of the few varieties which is on offer in both the standard 12-pack (pictured in the linked to review) and a 5-pack (pictured above).
The tricky question is how to get your hands on these if you are not in Japan. This is definitely a difficult one. Some online sellers, such as the Asian Food Grocer, occasionally carry limited edition flavored KitKats, but not the regional ones. You can get a regional flavor when it is issued as a regular bar if you get lucky. Rakuten also occasionally sells some KitKat specialties, but right now most appear to be sold out. You can get a Japanese KitKat ball marker from them for an exorbitant price, but that's a bit off the topic.
"Adzuki sandwich" KitKats ("Ogura Toast" from the Tokai region)
Frankly, what I would recommend to anyone who wants these is to try and set up a food exchange with an ex-pat living in Japan or a Japanese person looking for a penpal and a cultural exchange. Many people pine for certain specialty foods from back home and may be willing to do a care package exchange and some Japanese folks are happy just to make a friend in another country or to practice their English.
Kobe pudding (purin) in a special souvenir box representing housing in a historical district (ijinkan) in that area. Note that this flavor was once issued as a big bar and a mini, albeit in milk chocolate rather than white.
I know that there are some Americans who will send you Japanese snack packs in exchange for Reese's peanut butter cups, licorice, and Cap'n Crunch (all of which are hard to come by in Japan). As for how to find such folks, well, that's another issue entirely. You'll have to try out forums for foreigners in Japan and see if anyone has an interest. The main difficulty in such exchanges is that you cannot send anything via seamail from the U.S. anymore so postage rates are a huge chunk of change. You would need to negotiate an exchange value that incorporates that disadvantage (or both agree to pop for airmail across the board).
Beniimo (purple potato) KitKat representing Kyushu-Okinawa. Love this box design! Though I never tried this particular variation on sweet potato, I did sample two other sweet potato KitKats.
From my point of view, and I know that I've had years of experience and am quite jaded, I don't think it's worth the trouble. Yes, it is a little thrilling and cool at first to try these things, but after awhile, the weird KitKat flavors all sort of blend together. In terms of long-term enjoyment, I'd recommend going to Cost Plus World Market and finding their British or Canadian bars (hazelnut, orange), and just having those. What they lack in freaky coolness, they make up for in flavor staying power.