You know the story, Kouji Kabuto's grandpa has built a giant robot for him, called Mazinger Z, and gave him the choice of either doing good or evil, to become a god or the devil.
But if you don't, you shouldn't make this your first contact with the series. While it's a fun read for veterans on the series, if you're getting into it for the first time, you can get really confused, as many things are simply dropped on your face without any explanation.
The series stays in a steady pace for most of its run, but in the last two volumes you can tell
the author was either tired or wanted to focus on his other works, rushing for it to end, only to stop it abruptly after an event that while may be the climax, it's also nothing close to a conclusion. After it, you have a string of episodic short chapters to wrap up the final volume. You could read it as him just giving up of making this a series and instead just him trying to promote the anime, which had recently started, and its original concepts.
One could call it dated, but Go Nagai's '70s art style is so charming and dynamic that you can't help but love it, the fight scenes are well executed and with the poses he draws, it really gives you a sense of movement.
None of the characters are actually fleshed out, but they're mostly unique and memorable. Kabuto may be a pretty generic mecha protagonist from the early days, but characters like Baron Ashura and the other villains keep the series fresh and enjoyable. Also, the manga has a few bad jokes here and there, so you may either chuckle a little with some of them or hate them.
TL;DR it's a fun manga with a not-ending. You either like it or hate it.