Short review:
Well, this was something for sure.
The characters are surprisingly human and smart in certain aspects, and that is nice, but if I had to describe this story in just a few words I would describe it as an incredibly bland story with some shock value and some interesting moments.

All in all it's not great, but not terrible, it's just bland. You're not missing anything by not reading it.

Longer review, spoilers ahead:

As always I will be going straight to the point with the good and the bad:

This manga has some very nice art.

We have an intelligent main character, the male detective Mizoguchi, and an intelligent antagonist. This makes-up for an interesting dynamic and is more or less what carries the whole story.

Our secondary main character, the woman detective Inoue, feels more like a mascot than an actual main character, until a sudden change reaching the end of the story. Sadly, this change is fueled by love for the main character, which turns her evolution into yet another manga cliché.

The manga has an identity crisis. It does not know if it wants to be noir, lighthearted or mysterious. It tries to do everything at once and in doing so comes-off as silly at certain times. On the other hand, it provides an interesting dynamic at other times.

The fact that Mizoguchi is intelligent but also happens to be obtuse at the sight of technology adds a certain charm and some old-fashioned realism to his character. It also opens up an interesting dynamic between his character and Inoue, where he must rely on her for things such as operating a computer. At the same time, if you look carefully you can notice that the Mizoguchi appears to be more than capable of doing these tasks on his own, and instead choses to rely on Inoue to either prepare her for the field, train her, or just sighly annoy her. These two characters and their dynamic is a very strong point in this manga.

The manga has some of the most uncomfortable 'shoking' scenes I have seen. Not because they are particularly gory, but because of how removed from human nature they are and how relatable they are.
As an example, in a panel we can see a woman that has been sitting in her room for days letting mosquitos bite her non-stop. She just sits there letting all of this happen. In another panel we see a woman cutting a sea pickle, only to keep cutting until she sliced through most of her fingers. Whether these are a positive or a negative depends on the reader.

The main villain has a very good reason to be doing what he does at the start of the story, however after his main goal is accomplished, the character loses it's shine. The character needed more exposition for this story to work properly after his main goal had been acomplished, as it stands now his course of action from point A to point B feels sudden, and without proper exposition.
This manga is plagued with similar exposition problems, as well as the writer forgetting of a plot-thread they just started.

As an example of a plot-thread that was forgotten, at some point Mizoguchi sees someone suspicious in a taxi. He sends a police officer to find-out where the taxi was headed by calling the taxi agency and asking them directly. This thread gets shoved under the rug by the next page and never addressed again.

The pacing of the story is good at the start but as the manga progresses it hastens to the point where big moments in the story lack the proper punch and or buildup. At times the effect of this is so anticlimatic, it's almost funny.

It is clear that this manga was cancelled and the writer did his best to 'give it a proper closure', and I applaud them for that, but it is clear that this story required way more breathing room for the plot points to be properly unwinded.

Secondary characters do not ever get enough proper exposition, nor are they expanded much on at all. Most secondary characters just feel like furniture.

We get a suspenful scene where Inoue is seemingly about to be attacked by a bad guy inside a manhole. The suspense in this scene is great but... the conclusion is very anticlimatic.
Nothing happens in the scene and the bad guy was not relevant at all for this scene, he would only become relevant much later on the story. The story could have played exactly the same without this little suspenful scene and nothing would have changed at all.
The story is plagued with problems such as these. For example, the name of the story is 'manhole', and at the start we get a lot of mentions about how mosquitoes reproduce in manholes and the such, however, nothing ever comes out of this, which is sad because it reeks of wasted potential.

The ending is so anticlimatic is almost funny, and the way the main antagonist does a V of Vengance monologue before dying in a far-fetched manner is really bad.