I will write my review as an explanation to my rating.

Story: The plot, as simple as it appears, gets much more complicated as you read this manga. It tells us a story of parasytes along with the story of humans and the two are successfully merged together in a thrilling action plot. While this may not be the most original piece, it contains key contents that make you want to keep reading once you start.

Art: The drawing in this manga is not a 'standard' anime-like drawing you would see in Magical x Miracle or Mai Hime or most of other series. The drawings are more human-like and realistic. Sometimes rough, sometimes practically sketched, while sometimes quite detailed (Certain gory parts where quite visually descriptive) This kind of style suits this manga the most - keeping us reading a sci-fi manga while keeping us in touch with real world.

Character: Character development was great for the main character - throughout the story we see him change, we get to know everything about his family, relationship and the way he feels and thinks. The only thing that main character development lacks is his friends. We see him interact with them, but the story doesn't seem to talk about them too much. Which could be because the main character doesn't want any friends.
Another lack of character development shows in an incomplete storyline behind main sub characters (Arch rivals, girlfriend, etc). While it does not have an effect on the story - the fact that it wasn't there remains.

Enjoyment: I've already mentioned that this is not the first time that a story of aliens invading Earth comes out; however, once you get into it - you just want to keep on reading. The story sucks you in emotionally, bringing you a variety of feelings. It's genre also varies between gory horror to boring school days to romantic encounters to thrilling action.

Overall: All that can be said about this manga is this - It may not be a 'must read' because it is not as famous as manga that has been adapted into anime, but this manga is definitely worth reading (And a must read) if you like intense and anxious situations (This book has plenty). All fans of sci-fi will find this manga interesting, especially those who always enjoyed parts of movies like Spider-man or X-men when people just discovered their abilities.