This is overall a great show with deep philosophical implications and you should watch it just for that however it has extremely rough spots that almost had me give up. This review will focus on these negatives to give you an idea of whether or not you think this will be worth watching but please remember the lengthy discussion on negatives alone does not mean the show is overall bad. There will be light spoilers regarding these negatives which might be a good thing because knowing how bad it gets beforehand might lessen the impact and make the show feel better overall.
Firstly this show sometimes indulges in unnecessary violence, people get killed and mutilated even after the point has already been made. Yes sometimes shock value and gore can drive home the message but there comes a point after which piling on more bodies no longer improve the plot in anyway. If a bad guy has already been shown to be the worst because he killed dozens of people including major characters, why do we still need a couple more seconds to insert more random civilians to add to the kill count? The show also contains so many short scenes where random parasite infected guy picks up random women to feed on them, yea ok we got it already the first 5 times we saw this, no need to repeat for the 6th time!
Near the mid point of a show we have a series of major plot points driven by the same "girl being incredibly emotional/stupid" gimmick. There's this one minor character Yuuko she suspects one of her classmates is a parasite so she follows him around, seeing his head deforming in impossible ways, seeing him literally shape-shift to another person, talking to her brother who is a police and knowing about the existence of shape shifting parasites, they eat people, and are connected with the recent serial killing around town. Knowing all this, what does Yuuko decides to do?
She asks the guy out ALL ALONE , to ask if he's a man eating monster.
This has got to be one of the all time dumbest characters and it was all entirely unnecessary, the plot could've easily happened exactly the same without Yuuko being unbelievably stupid. I was so convinced when I saw this that this might be one of those weirdly sexist shows or something but not really when you look at the big picture. For some reason they just decided to pile on "girls being stupid" within those couple episodes to drive the plot and it was extremely rage inducing.
I decided the show wasn't sexist because highly variable intelligence/competence becomes a theme and show up multiple times with male and female characters for the rest of the show, including the protagonist. I appreciate the message being delivered by the plot going the way it does but I don't appreciate how dumb characters are often required for that to happen. For example the MC is said to have various superhuman senses he can hear someone breathing from far away in the middle of the city. Yet when a bad guy shows up to massacre people in the same building, he runs around like a headless chicken unable to find where this massacre is happening for SO LONG, because the plot needed the bad guy to kill more people. Which also feeds into the first problem I had. They dumb down the MC so the bad guy has more time to kill random no name characters.
The MC also goes through an arc where he loses his empathy and becomes more calm/logical and calculated, which powers up his combat potential. However he still randomly loses his cool when the situation requires it, there's just no consistency. When his girlfriend shows up he loses the emotions so the girlfriend can be upset and like hey are you still human are you still the guy I had a crush on? But then when he needs to fight he suddenly becomes scared/worried/agitated and can't execute the most tactical decisions, so that the bad guy can do more damage.
We have some characters that may at first appear to be important but ultimately feel pointless. There's a guy who only exists to bully the MC, gets beat up a few times after the MC gets his powerup, and he just disappears. We don't know what happened to him we don't know if he's learned anything other than he can't beat the MC in a fist fight, he's just forgotten for the rest of the show.
Despite all that like I said the show was good because the plot ultimately did have important things to say. For example you've surely heard of the argument that humans are "bad" because we pollute the environment and stuff. But so what? Who actually cares? Animals don't care because they don't understand this is going on, the planet doesn't care because it's a chunk of rock it doesn't have feelings. The only ones who care about the environment is actually, us. This is one example where complete self-interest can logically lead to some kind of common good, and there are more questions like this in the show.
The philosophy still isn't bullet-proof, for example the parasites are said to be emotionless logical creatures that prioritize their own survival over everything else. But why do they eat people? They don't have to, they can eat normal food. Maybe they find humans the most tasty but "more tasty but with risk of dying" is surely not the logical thing to do.
I also like the show for at least having the Parasite and human factions as a whole be intelligent and competent. The parasites quickly come to the conclusion for example, randomly killing people is unsustainable. Humans on the other hand also quickly capture parasite samples, deduce their weaknesses and deploy counter-measures, which make for exciting developments in this secret war, even if individual characters sometimes make stupid decisions.