One of my favorites as a kid, then I grew to really dislike it. However, upon viewing it again recently, I liked it more than I thought I would. In short, I really dislike the excessive use of stock footage, and the big fight at the end is the weakest part of the movie for me. The idea of Godzilla being threatened by a circle of fire is just ludicrous. Why couldn't he just walk through it? Not a very convincing way to create a sense of peril for our heroes, though I do love the way Megalon and Gigan's moods change clearly from joy to disappointment once they escape. I also think Gigan looks a lot let impressive in this one. And little Roku-san (for the longest time, I thought his name was "Roxanne")...bleh. Seriously one of the most annoying characters in any movie.
That being said, I do find some genuine enjoyment in the film. The way the story clips along is fairly exciting. The design for Megalon is pretty interesting, and Jet Jaguar just appeals to me for whatever reason. The music, excluding the material used in the final battle, is terrific, with a great mixing of styles that works better for the film than one might expect. While the effects are generally rather weak, the scene of the lake drying out at the beginning and the dam burst later on are pulled off beautifully.
One question: What was the deal with Hiroshi and Rokuro using a model plane to take out the Seatopian agent? If their goal was to take him by surprise and knock him out, there are plenty of other things around the shop they could have used without going to the trouble of making a trip out to a hobby shop to take a model plane.