My first exposure to this movie was also my first exposure to Famous Monsters Magazine, issue #91, we were on a cross-country trip and pulled off the highway somewhere in some bible belt state....
Somehow Dad let me have the magazine (strange, considering how much he hated me liking this stuff....my stepmother hissed "it's the work of the DEVIL!" shortly before I moved away from them permanently
)
It would be another 4 years (1975) before I finally saw the movie on afternoon tv in L.A. after school (same channel that introduced me to Gorath, Atragon, and other more obscure Showa Japanese movies), and man what a shock....growing up to the distinctive atmosphere of 60's Toho, I was expecting and envisioning that, and got....this.
I was still learning who was who in these movies at the time, (and the limited sources of information at the time definitely had a condescending bent towards Japanese sci-fi) how they were made, the concept of stock footage, waning budgets, but the jarring shift in atmosphere and tone was kind of short-circuiting; expecting to hear Ifukube's music, see Tsuburaya's effects (still too n00b to understand that he had passed away) and getting...this.
That music, I couldn't get that music out of my head.
It did help illustrate one thing, and that was the direction the movies seemed to take after 1970, and I realized then how much I loved the 60's movies.
Maybe it took the Heisei series to revisit this and have a new appreciation for the 70's G-movies, I dunno.... I still cannot sit through Megalon. But I look at the 70's G-movies these days as an offbeat guilty pleasure. Actually, I love GvsMG and ToMG, with this one close behind, and as much as I love the monster Hedorah itself, the movie takes some effort to get through.