The tubing quality seems as good as the British, Italian, or French steels. Construction quality of production frames varies from builder to builder though.

For the really nice, "world class" Japanese frames, look at Toei, 3Rensho, Nagasawa, Zunow... I'm sure there are plenty more. They're built at least as well as any of the best around, but were sold as high-end frames (I think Toei is/was custom only; not sure). I recently sold a Panasonic "Team America Custom", built of Tange Prestige... It was very nicely made, but still a production frame (not really a custom). I also recently sold a Miyata Team Pro, and it too, was nicely built.