It's excellent and one of the big anime titles of the year. Despite gruesome appearances, I also don't think it's a horror series.
Hitoshi Iwaaki's Parasyte manga was famous twenty years ago (1988-1995), but no one back then adapted it for either the big or small screen. New Line Cinema bought the movie rights ten years later, in 2005, and then failed to get the project out of development hell despite the planned involvement of Jim Henson Studios. Nonetheless it sounds their failure was the indirect cause of multiple 2014 adaptations anyway. Their option expired in 2013 and sparked a bidding war back in Japan. Before long we had this 24-episode anime series and two live-action movies (Parasyte: Part 1 and Parasyte: Part 2). I'll be watching those too, as soon as I get my hands on them.
The story involves parasites, of course. They invade human bodies and turn us into man-eating flesh-shredding monsters from a John Carpenter film. Your head will split apart into bladed tentacles three metres long that can bisect a car. Killing a parasite is tough. You either have to destroy its host body or else pull the parasite off somehow and keep it from returning or finding another host until it dies. Since the parasites generally eat their host's head and squat there in its place as a head-substitute, in practice your options are either decapitation or destroying the heart. (The parasite can simply heal any lesser damage.)
Personality-wise, the parasites are cold-blooded predators without empathy or emotion. They're smart, though, and they learn. Also a major element of the series is what can happen when the line between human and parasite gets blurred. If you're sharing a body and a cardio-vascular system, you're going to... drift. We see this in both directions.
The protagonist is Shinichi, a meek schoolboy with the air of a bookworm. One of the first things that happens to him is a parasite drilling into his body and burrowing up towards his head. This fails because of quick thinking by Shinichi, so the parasite accepts defeat and eats his hand instead. Shinichi now has a talking alien hand with a habit of growing organic sword blades whenever it feels threatened, although the good news is that it's not carnivorous. Instead, Shinichi's now eating for two. He calls it Migi (i.e. "Right"), because parasites couldn't care less about human things like names, and the two strike up a working relationship. Shinichi must promise to do nothing that might endanger Migi (e.g. alert the authorities, tell anyone about him, etc.) and in return Migi won't massacre all his friends and family.
You'd expect this to be horror. Body-warping freakazoids are killing and eating people on-screen. Personally, though, I don't think it is, any more than the presence of zombies would mean we could only be watching a horror film. Horror's about scares. That's not the goal here. The subject matter is gruesome, yes, but the show's not trying to do horror movie scenes. Even when Shinichi and Migi are facing an apparently unstoppable opponent (e.g. Gotou), this is played as character exploration in a last-ditch situation, not for instilling fear in the audience.
Instead, I think it's science fiction. The show's exploring themes, ideas and characters, with an occasional environmental angle and a fair amount of pessimism about where the human race is going. Might the world be doomed if mankind didn't have any predators?
It's clever. It has rules for how its parasites work, which it maintains scrupulously and yet still keeps finding ways to surprise you. I love stories that explore themselves this thoroughly, with the parasites' plans and tactics making a lot of sense.
It's moving in unexpected ways. Shinichi's increasing distance from humanity will cause him distress, but more startling still are the rare parasites who in time discover altruism. This show might make you cry for a monster who's killed and eaten 38 people and is struggling just to understand its own purpose.
Sometimes it's even funny. Migi is occasionally curious about Shinichi's sex life, which is not a good thing. The jeering, wanking psychopath is a hoot, until suddenly he really isn't. Tamura Reiko's experiences with motherhood can also be amusing. She's a parasite who decides to get pregnant, just to see what might happen. Not Like Most Mothers.
The female characters appear to have caused some mild fan controversy, although I can't say I really had a problem with them. Part of that reaction, I think, might be simply the lack of female cops, soldiers, politicians, etc. None of the female humans here can fight a parasite. They're not superpowered like Shinichi and they're not Men In Uniform. I don't think that's a problem, though, and in any case we have two important parasites, one being SPOILER and the other being Tamura Reiko. (I presume she hadn't been inherently female before fusing with her host body, but she's gone the whole hog in embracing her adopted gender as well as being the cleverest person we meet of either species.)
Meanwhile among the female humans are some schoolgirls. Shinichi goes to school, so of course we were going to meet his classmates. They're normal kids. Not being monster hunters, they're thus liable to be left on the sidelines. Some still manage to get some plot involvement, but this isn't invariably a good thing. A girl who knows about parasites has realised that someone she knows is one of them, so she decides to... (wait for it) talk to him. On her own. Without telling anyone. The alternative would have been to tell her policeman brother who's investigating them and has already discussed them with her. D'oh. As it happens the impossible happens and she survives, but (a) that doesn't make her original decision less stupid, and (b) seventeen other people aren't so lucky in the ensuing massacre.
Girls also have a tendency to show romantic interest in Shinichi, which can get a bit tiresome for the audience if you're not feeling indulgent. This isn't that kind of show, while Shinichi himself is more interested in heads being bitten off. (That's arguably an unfair reaction, though. It's based on the genre, not the situation. The girls don't know they're fictional characters and I can imagine Shinichi being an attractive guy, especially after becoming calm and intimidating with the power upgrade.)
Oh, and there's also Shinichi's sort-of girlfriend, Murano. She can look like a slapped fish and has an occasional habit of asking if he's really Shinichi. However the former isn't her fault, especially under the nightmarish circumstances, while I personally see the latter as a sign of keen perception. I liked their pair's relationship. It's not a conventional romance. Shinichi's often driving her away with his increasingly off-putting attitudes and priorities (e.g. "Get out of the way, humans!"). There's genuine uncertainty about whether she'll end up dead or fleeing from him in horror and/or despair. In fact, I'd say this feels more grown-up than most fictional romances. Their relationship feels fragile, but it can move faster and further than stories where the romance is more central. They can surprise you. If two such people were really in this situation, I can believe that they'd behave like this.
It's very, very good. It's full of interesting themes and ideas, from the meaning of motherhood to mankind's place in the natural world. However it's also a kick-arse thriller with anthropophagous tentacle-headed walking nightmares and a hero whose right hand can extend three metres from his body and decapitate you. Shinichi and Migi's relationship is the most important in the show and it's always great. An evil (or at least ruthless and empathy-proof) parasite is sharing the body of a well-meaning human. They don't trust each other, but they get on surprisingly well. They have to, really. Migi's oddly cute. Sometimes he's even funny. I'm terribly fond of the callous little monster, actually. If you offered me a Migi-hand, I'd be tempted.