Produced alongside the anime variant, this manga is just that, a variant. Not to be confused by the synopsis quote "Which goes BEYOND the plot of the TV series!". It's a story that goes beyond it fully, being a new story all to itself, but still based on Keiichi Sato's concept designs. If you're starting it with the impression of learning more about the anime you may end up, dissatisfied. With that cleared up I'll attempt to review The Big O without comparing it to the anime.
With the same plot, it features Roger Smith. The top negotiator of Paradigm City, a city which lost it's
memories forty years ago. The premise surrounded in mystery is one that's prominent throughout the series but not stressed with importance. It's like the saying goes, "The journey is more important than the destination". Being shrouded in mystery the series has quite a bit that isn't explained or told straight to you.
While this could be viewed as a bad thing, it's a unique approach. One of which I both liked and disliked, probably because I'm use to knowing the truth behind everything. These holes fit well with the setting in that no one remembers much from forty years ago. Also it limits and humanizes the perspective of the reader. Instead of being the all knowing god with everything explained, you're left feeling much like the characters in the story, confused and unsure about things. Though luckily for you, you remember who you are.
On the topic of characters, The Big O manga has a good variety of them. From the cool and confident ladies man Roger Smith. To his diligent and loyal butler Norman. The antagonists as well have traits specifically only to themselves, which adds a good amount of individualism. They aren't your generic evil or corrupt individuals bent on seeing others suffer. No they each posses their own agenda and have distinct ways of going about achieving their goals.
The pacing of the manga is a bit weird for me personally. It might be because I read it too quickly or my inexperience with reading manga. Broken up into chapters it usually begins and completes stories between them. Sometimes introducing new characters without much back story, it at times seems to rush the plot forward. Though I attribute it to the mystery aspect, in that you aren't explained everything to begin with.
As for character development, there isn't much. It's more along the lines of character expression. You slowly get introduced to the personality and thoughts of the characters through their actions and dialogue. This spans the majority of the manga even up to the very end. I enjoyed this representation as it made me feel more like a bystander watching instead of being inside the mind of the characters.
The mecha or megadeus as they're referred to in the story are unlike conventional giant robots. Not exhibiting ninja-like speed nor grace, they're metal behemoths that rely on brute strength and "old school" weaponry. Their purpose and creators a mystery to those who control them. Though this hardly stops them from duking it out in the streets of Paradigm.
Finally the art of the manga which is a critical part was overall, really good. As I've said I don't have much experience with manga, so put beside newer and more well received manga it probably doesn't compare. Though for me I quite enjoyed the art and it's "Noir film" and "Batman The Animated Series" similarities. Which apparently was a big influence for the series as a whole. I've read many times where The Big O has been labeled, "batman with mechas". While very similar I'll agree, it still holds a uniqueness that makes it one of a kind.
In summary, it's a good short series to read if you're into noir, mecha, or mystery. I enjoyed all of these aspects and how they were brought together. It stuck true to Sato's concepts and did a great job. A true classic for what it strived to be at the time and for it's presentation.