Higanjima is a horror/action manga about a bunch of people going to a island with a bunch vampires. I read almost half of the manga so my insight.

Let's first state the vampirism rules from this story: The vampires aren't killed by sunlight and only can be killed damaging their brains. To became a vampire, you must be infected by their blood. Vampires need blood so they can halt a mutation that can transform them into demons (A powerful creature, but without inteligence). They apparently have bad eyesight and good nose, but, besides the beginning of the manga, this never is brought up again. They also can make their hair go white if excited, but we never see this apart from the beginning. (Considerating all vampires also wear hats or are bald, this it's difficult to know by certain)

Despite the manga being labeled a seinen, the plot really like a generic shonen. I guess is just a seinen because of the violence and nudes.
It's pretty much, our main cast trying to defeat our main antagonists.

The art is nice. The gore stuff are very detailed and I love the design of the demons and vampires, despite being kind simple, it's creepy. I think very noticeable is that the mangaka has probably issues to draw the caracters differently. You can see this when the main cast spend 8 months in the island and apparently wear the same clothes or Rei and her CannotRemoveMakeUp wearing the same outfit even in flashbacks! (Kind funny seeing everyone wearing kimonos and she's there wearing a jacket and dress).
All background characters are just the same character draw over and over.
Also another issue is, because this a seinen you'll see a lot female nudes, but is it really necessary show, everytime a vampire bite someone, the person pissing themselves? I get the point in the first times, but this happen everytime! I think the mangaka has a piss fetiche.

The Main Cast is pretty much:
Akira, the main character, his job is to be Emiya Shirou and constantly cry because he want to save everyone, but he can't. Interesting thing: in the beginning of the manga is stated he's very smart and very perceptive, he can pick up stuff that no one can't (Like knowing if someone is lying or predict the vampire would escape the car)...this is never brought up again.
Kato, he's the brawl, but he never fight against the vampires so he's useless.
Nishyama, he's the brains, he's job his to give random informations for the reader.
Ken, he's the comic relief...
Yuki, she's the girl of the story. Her job is to be the Damsel in Distress.
Pon, he brought a gun and give character development to Akira.
Atsuhi, he's the badass brother. He's there to save everyone, because everyone is useless.
Miyabi, he's the antagonist...He's evil.

The character don't have much deepness, they are pretty much how I described them. No one actually has a character development or get strong, besides our main character.

Another issue I have with this manga is every chapter he does a recap of 1 or 2 pages about what happened in the previous chapter. It's very weird if you have the physical copy and you see the previous chapter on the right and the next chapter on the left. You can even notice he recycle some panels!

Sometimes I think the mangaka think the reader has amnesia. In some point of the plot, it's revealed a character became a vampire. Problem? We already knew about it, but the manga treat this like is some big revelation. Even the main cast treat like it was a big revelation despite they saw the character being infected!

Well, my overall until now is 4. If you like high violence and cool action scenes you will probably like more.
Despite this, I'll still try to read this. I mean, I have no hope the manga will get better, but I want to see how much the mangaka can stretch this story. It's 33 volumes, and this is just season 1! It also have a sequel with 16 volumes and another sequel with 14 volumes until today (2018)!

PS: This manga make me research if Katanas can cut metal. ProTip: They can't.