Frijoles in Azabu-Juban, as featured in the Japan Times today... The first specialist taqueria in the city and it's fine. Flavors are a bit toned down — this is Japan, after all, and palates are not as inured to capsaicinoids. And it's true the salsa is anemic. But I still give Frijoles a thumbs-up. And even if it's not worth travelling across town for, that may not be necessary in future if they spawn franchises (as seems apparent from their web site here...)
Here's a map link...