I'm currently making my way through all the Godzilla films, and I picked this since it had been coming up in the forum lately.
Even as a kid, I remember that I never really liked this movie. It's an ok G film. The music is downright awful. People can defend this film up and down until they're blue in the face about it having a low budget, but how exactly is that a reasonable defense of overused stock footage and poor script writing? Somehow Seatopia knows Space Hunter M and sthey're magically ok with sending Gigan...only it's halfway through the movie and whimsically introduced. Megalon attacks planes and then Gigan's arm is shown destroying them. Some of the stock footage that is inserted is completely unecessary. In example, why bother showing the G suit used in Gigan for 1 second when Godzilla is trapped with JJ in the fire? It adds nothing and completely distracts the viewer. I understand limited resources mandated stock footage being used, but it isn't even done in a reasonable manner in some cases.
And seriously, that score....it sucks. Kids movie or not, it's almost headache inducing.
Now I will point out there are some good things in film. I love the countryside setting and the set looks great. There is something to be said about the way the Showa series brought these sets to life. The Dam sequence is wonderfully done. Gigan and Megalon (really G and JJ as well) have tons of personality, even if they're dopey to a fault at times. I really like the way this film uses quick cuts during the fight scenes. The close in shots make the fight move quickly and for the most part it's done very well. The ring of fire is cool. I like Megalon's napalm ability.
But all that aside, it's not a great movie, but I can have fun with it.