Toho was totally on fire for a brief period in the early to mid 60's. Though even later they sporadically came up with something great like WAR OF THE GARGANTUAS or BATTLE OF THE SEA OF JAPAN, most of their best film were actually non-Godzilla films.
ATRAGON is one of the first and best examples of 60's Toho lunacy and excellent Tsuburaya special effects, combined with excellent acting. There's plenty of absurd science, ridiculous ideas like flying subs, massive destruction, fast-paced action, and even a giant monster to keep anyone's attention.
The only thing weighing this film down is the pacing being totally off. There is waaaay too much time devoted build-up and fleshing out too many 1-dimensional characters. A lot of sequences (like finding the legendary lost captain on an uncharted island) are way too drawn out and sap the film of a lot of excitement. The monster "Manda" is poorly animated and underutilized.
That said, the city destruction presented toward the end is some of Toho's best, with a massive crater opening up under the city and sucking buildings down several hundred feet, presumably killing thousands! Another highlight comes with the Mu sub going nuts on Tokyo bay destroying several merchantmen over and over again. It's colorful fun all the way which refreshingly can appeal to children and adults alike. Toho's later scifi films tended to try too hard to stay kid-friendly.
With a little editing and a few script rewrites, this could have been Japan's definitive sci fi epic. As it is, a flawed but enjoyable romp through Godzilla territory sans atomic lizard.
