I must preface this review with the fact that I have not read the original light novel which this manga is based off of. As such, I suspect many things I say here about the pacing and plot may not be the case for the light novel. I highly suggest reading it as I believe that this story is an interesting read (though I have not read it myself, but I do intend to!). Anyways, here's my review for this manga (see the tl;dr at the bottom if you don't want to read everything). Minor spoilers by the way.
Clockwork Planet takes place in an alternative reality where planet Earth was effectively replace by a mechanical world created by an enigmatic person known as Y. Our main character, Naoto finds himself entangled in the mysteries of his world and effectively goes on a journey to save his malfunctioning planet.
While this is by no means a unique premise, the author was quick to establish intrigue for the world. This is then layered by the introduction of our main character and the main heroine, Ryuzu. The initial stages of the manga introduces our MCs and the fantastical pseudoscience that governs the laws of physics, which hooked me immediately. As the story goes on the plot gets greater in scale and covers the politics of mechanical Japan as well as the organizations and people that govern the cities that are actually built on top of giant gears. Unfortunately, I must admit that towards the end of the manga, things get a bit nonsensical, but this is mostly the fault of the main villain who is effectively the big bad final boss for this manga.
Out of all the characters, the main villain is the one I have the most problems with. He's shown to be an insane character, but his backstory and motivations feel shallow because of how little we get to see him. This character tends to ramble about things we the audience never actually get to understand as the manga is cut after the final battle. This is a problem I see with many villains for this series, but the main one is by far the worst in my opinion. On the other hand, our main cast and supportive cast is very well put together. Despite being the generic protagonist Naoto manages to express his own opinions and motivations while being an effective, self-insertable character. He's not overpowered in the traditional sense, giving him some sense of vulnerability. His "wife" Ryuzu, is visually my favorite character, but after grabbing the attention of the audience plays more of an comedic, yet overpowered plot device that drives the initial and later battle scenes. While her motivations are generic, she makes up for it in story driven beats. Marie is your classic tsundere. Many find her annoying, but I think she's pretty likable. Her bitchy attitude makes sense given her background and she rounds off later in the plot. Her partner robot, Halter is first introduced as your classic bodyguard. He doesn't stand out for the most part, but shines toward the really end of the manga. There's also the supportive cast (which is really just three characters) and they are enjoyable in their own right.
The artwork is clean and dynamic. Character designs are directly inherited by the light novel and they all look really cool and intricate. If you ever get to see the coloring for the characters, they all stand out yet complement each other. It's like watching a rainbow in front of grey clouds.
TL;DR
I personally enjoyed this series for the character dynamics, world building, and the cute characters. There were many comedic gags that I thought were cute despite the main story being somewhat dark. While I gave this a 7 overall, this should not dissuade you from giving this manga a try. If you came here after watching the anime, just go read the light novel!! I will soon do so as well and I am pretty excited to see where this series may go. Thanks for reading and have a good day!