Obviously, this movie was not made for the kids who grew up with CGI. Anyone with a keen eye for special effects can see how it is done. But in those days, there was only a handful of techniques you could use. This is miniatures and stop motion, and although not in the league of Ray Harryhausen, there is far worse.
Some people claim it is a BEAST FROM TWENTY THOUSAND FATHOMS ripoff, a mock buster. I don't agree. All these movies have a similar pattern. Something inexplicable happens, slowly the mystery and the number of incidents grow, a scientist comes up with a possible explanation and then the grand finale where the special effects and the military come in (with lots of - sometimes inappropriate - stock footage).
The Giant Behemoth follows that pattern. It takes half an hour before the monster is shown and then you get a good 20 minutes of more than passable special effects. No men in rubber suits here. The scene at the power plant is a classic as far as I'm concerned.
For the modern viewer it takes a bit of patience to chew up the first three-quarters of babbling around, but there is enough to keep your attention. Not like It Came from Beneath the Sea where the continuous scenes on the submarine gets you craving for the end.
All in all, it is a pretty decent movie. There are far, far worse.