Indeed, whatever shortcomings this film may have, the effects work is not among them. not only is this colorful film a sumptuous visual treat on the order of the first Mothra film, the model work is excellent, with a variety of beams and rays (prefiguring the Heisei era to come), titanic explosions that really tear the refinery to bits, emphasizing Mechagodzilla’s power and creating a great battlefield of flames for his first confrontation with Godzilla, and various locales from cities to the countryside (with crumbling cliff) to a storm-wracked island to a subterranean base, are all convincingly staged. If there’s any weak point, it would be the aforementioned human-to-ape-alien transitions (sometimes accompanied by the needless and gruesome spraying of black blood)- but even they had an icky, grotesque effectiveness with the liquid-effect covering the stage-transitions of makeup.
All of this I agree with in particular. One of the film's great qualities no matter how unsure it may feel in other aspects is its sense of spectacle. I feel it nails that the best of any of the '70s films, and ranks among the best in the entire Showa series.
The human scenes are, unfortunately, a bit less tightly constructed than Honda's best entries. They're serviceable in vs. Mechagodzilla, but not as colorful and spectacular as the best special-effects sequences in the film, nor is the framing as clever. Honda snuck some unexpectedly excellent shots into Mothra vs. Godzilla even when there's no monster on screen.
Despite any of its shortcomings, though, this is never a film I can get through without smiling. I've only gotten positive reactions from group viewings too. And not with any sense of ironic detachment. It's just an infectiously good time.