Plot
The biggest downside of Ajin is its story and how the narrative plays out. In general, this manga likes to use the "rule of cool" but does not do it effectively enough to warrant this level of storytelling. Throughout the story of the manga, you will experience hiccups in the writing and the reading flow will be disrupted, leaving you questioning if you missed a chapter. Furthermore, some decisions are plainly made to advance the plot even though they make no sense whatsoever. You might scratch your head or roll your eyes, but it is not too bad. A few psychological and moral questions are presented but they are no real conundrums, pretty much every single one of these questions has been tackled and answered before.
Compare Ajin to an action movie, it is enjoyable and entertaining on some level, but it won't blow your mind.
Characters
This is a mixed bag. On the one hand, the characters are nothing new and rather mediocre, on the other hand, this is not a bad thing for this type of story.
A lot of it is told and pushed forward through the main character, with side characters occasionally contributing a small part.
A decently sized cast is introduced throughout the manga but no one really sticks out too much. Most of them are quite one-sided with no character depth or development at all. The main antagonist is the most entertaining of the whole bunch, but that is not saying much.
By the way, if you like smoking characters you will fall in love with this manga. For some reason, everyone smokes non-stop. Honestly, this manga could be an ad for cigarettes.
Art
Now, the star of Ajin. Its art. It is clean, easy to follow and does not get stale at all. A well-balanced mixture of simple and detailed panels from start to finish. The art adds a lot to the overall enjoyment and helps considerably with telling the story. Especially action panels, which there are a lot of, benefit greatly from the art style.
Overall opinion
As I have mentioned before, I think of this manga as a decent Hollywood action movie. The story and characters are plain average, maybe even lackluster if you are being critical, but it is still somewhat engaging because of its art and the "rule of cool". Don't expect too much and you will be fine.