Great series. Great characterization and an engaging plot. Even if characters often take on stereotypical personalities, they're treated with a degree of respect that gives them a satisfying depth.
The atmosphere is really well done. Even if, individually, the ghosts or otherworldly creatures aren't creepy, the context and way in which they're applied never fails to at least put me on edge. It's this eldritch form of horror that has a real soft spot in my heart.
I'm really fond of this series. Just binged the light novel source, too.
The little I have to say here is just apparent of how good this series is.