It's no secret that Imagawa himself wasn't happy with how this film turned out, and it sharing its name ("Zangetsu" - the lingering moon) with one of the Magnificent Ten from his Giant Robo OVA has led some to believe that Imagawa just used this opportunity to animate one of the chapters that would have chronologically preceded the OVA ("The Birth of Zangetsu the Midday"), but replacing all of the characters with Tetsujin 28 characters. Shotaro's older adopted brother, while not featured in the original manga or any of its previous adaptations, does bear resemblance to some of Yokoyama's characters. The same goes for the apartment manager he lives with, who is based on a character from Sangokushi.
The backgrounds are as good as the show, though for a movie it's merely the expected standard, and the animation as well is decent but nothing noteworthy; character acting's almost nonexistent and the film only has a few robot fights (one of which reuses footage from the first, though for plot-related reasons). The visual presentation of the finale will be extremely familiar to you if you've seen Giant Robo.
Tetsujin himself is practically a background character, and while it does attempt tackling some of the same themes as the series, the short length prevents both the impact and depth it presented. Though I do appreciate how it depicted some aspects of the era not present in the series, like the unexploded bombs or repatriated soldiers.
If you liked Takamizawa and Kenji in the series, here they act essentially out of character, Kenji presumably because he hasn't gone through the trauma of having lost his brother (though he strangely still acts as a journalist for one scene), but Takamizawa is just inexplicable. The rest of the cast who do act the same as in the series lack depth across the board due to not having experienced the same events.
The antagonists' (there are multiple) motivations range from shallow to a tad incomprehensible, and some of them are just trivialized versions of the same antagonists from the series.
All this said, it's a passable film overall that I don't regret spending my time on, but there's no loss in skipping it.
