Giant Robo is one of the few works of legendary mangaka Mitsuteru Yokoyama that has been fan-translated in English, in its entirety. That, combined with my experience with a few "adaptations" of his work, is what lead me to give this, a late 60s, giant robot manga for children, a shot.
The manga was produced alongside the tokusatsu series of the same name (also known as "Johnny Sokko and his Flying Robot"). I'm not aware if the idea was initially made for the toku series, or the other way around, but that's irrelevant.
Giant Robo starts off, surprisingly, very exciting, with great pacing and an initially intriguing
plot: Daisaku Kusama, a tourist, finds himself caught up in one of Big Fire's grand schemes and inadvertently becomes the only person who can give orders to Robo. After the strong introductory chapter, it becomes very clear that Giant Robo is an action manga for children, as the last two "arcs" are fairly run of the mill stuff. It's entertaining, and a very quick read, but one wishes Yokoyama revisited the premise at some point and tried to make more of it.
It's worth noting that the fantastic Giant Robo OVA series has absolutely nothing to do with this manga, aside from the robots themselves, the concept of Big Fire, and the main character. The OVA is more of a tribute to Yokoyama's work than a direct adaptation of any of it.
Is Giant Robo worth reading today? Well, kind of. Not really. If you like old manga you might as well read this one at some point. It's interesting to see some of the roots of the mecha genre, I suppose. But the middling second and third arcs as well as the lack of any direct continuation ensure that I wouldn't recommend it to anyone else.