Sweet mother of God. This is, hands down, one of the darkest anime I have seen. But, I haven't seen a whole lot, so I can't judge by other than what I've seen.
Shiki is a beautifuly done anime that portrays a dark and engrossing story of survival on more than just one end. There are several characters to remember, the animation is great, and the music is spot on.
For story, Shiki has outdone itself. it starts with a mysterious occurance killing off villagers. What I like is that you find out what's going on quickly, but you still learn more as you watch. What this anime also does is that it plays with your sympathies a bit, and I loved that. You find yourself (I did at least) sympathizing with one group, but then sympathizing with the other pretty quickly once the shit hits the fan. You start to wonder who real bad guy is. It also has an open ending which helps conclude it decently, but could have had a hint at future prospects perhaps. But one can dream.
The animation is excellent. Well, the animation of people's tears seems a bit off to me. But that's really my only complaint.
The music isn't all dark and ominous. For the most part. What it is, is creepy almost childlike music that's light and quick. But it creeps you out a bit when things... happen.
In relation to the last two topics, the first thing that stood out to me was the opennings. I watched every single one because they enticed me so much. I don't know why, but the music and the animation in the openings synced perfectly and added another creepy element.
The characters are, for the most part, likable. There is one that I really don't like because he's a huge douche nozzle. But that's just me. Another thingis that you'll (I did) feel sad when most of the characters die. Especially when they die in acruel way. This happens much much more in the second half. Again, the shit hit the fan. But in a good way.
Overall, I'm not going to say this anime is incredibly scary. It's really dark and somewhat horrifying in that you can't believe that things are happening as they are. Especially during an experimentation scene. And the second half. Some of the deaths were just... cruel. And it made my sympathies switch sides very quickly. If you're into a slightly creepy, gorey story with some mystery thrown in, I suggest you check it out.