In honour of Gigan's 50th I watched this again a few nights ago, and I still find it enjoyable. The human characters (and cockroaches!) are fun, the story is just the right kind of crazy Showa Godzilla silliness, and Gigan is a really great creation. The pacing's pretty good overall too, though I do think it drags just a little bit during Gigan and Ghidorah's initial rampage, albeit not as much as I remembered it doing from me previous years-old last viewing.
On the negative side though it's painfully obvious that King Ghidorah is only there for some extra monster star power, because 99% of the time he just stand there totally stiff and lifeless, like they couldn't find another suit actor to play him nor anyone in the studio to animate his heads or tails. It's quite bizarre. Does anyone know the exact reason behind that? Was the suit just in such bad shape at that point that they didn't want to risk it completely falling apart? Either way it's a shame because his appearance feels completely unnecessary as a result, most of it being relegated to stock footage.
And speaking of the stock footage, it's a bit of a mixed bag. Stuff like the monster montage at the beginning during the discussion about Monster Island is totally fine, as are many of the shots of the military and some of the destruction. But straight-up recycling Ghidorah's fights from earlier movies only with a darker image to try and make it look like they're happening at night just.....isn't so hot.
But yeah, overall it's still a movie I enjoy, perhaps even more than I was expecting to. I also kinda love how you have one big "investigation" montage, then the characters all meet up and discuss their findings and are like "we need to investigate further!" and it goes straight into another montage!

It's also 70's as hell how many key parts of the film depend on characters being heavy smokers. It's just fun.
Oh, and I love how the main character is listening to the director of this "Children's Land" just casually talk about how they're going to destroy Monster Island and kill all the monsters at the beginning of the film and doesn't for one second think "wait a minute, how the hell is a dude running a charitable children's theme park organisation going to do that?!"

But in this kind of movie? Stuff like that just totally works!