Put me in on the side of those who like this movie. What David said is quite true. This was the first ever Gamera movie I saw, and the stock footage did not bother me one bit. (As a kid, I wanted to see monsters fighting, and that is exactly what I got).
In viewing it today, yes, I now think the stock footage is a tad too long, but looking past that, the movie is still enjoyable. The cool looking flying saucer, the aliens with their cold sinister stares (looking at this as a kid, it was quite alarming), and Jim and Maseo, who were certainly mischievous, but never ever annoying. Their exploring the Viras spaceship I thought was so cool. Nothing of that sort was even seen in the Godzilla series.