The Jigoku Shoujo is not classic horror as you might expect. Don't expect sleepless nights after watching this anime.
I consider Jigoku Shoujo as the best portrait of evil and hell there is. Yes, story is repetitive but this also proves point in the big picture - endless circle of evil. Also the main point of Jigoku Shoujo is that vengeance isn't justice and doesn't help you at all. And more importantly that hell is state of soul rather than place with fire. I mostly appreciate that hell seeks to destroy both evil and good without distinction. It's against stupid fairy tail folklore where hell behaves as some just punishment institution. Story is more educational and dramatic rather than scary and depicts the true evil. The first season main story arc is about Jigoku Shoujo itself and kicks in the last third of first season. Even if it isn't as good as the second season and the best and last season, I give it 10 without hesitation for reasons described above.
Animation is decent, but there is great deal of nasty repeating shots. I don't like this practice no matter how good this scene is.
Sound is good and music consisted of rock and classic is great. However the third season of Jigoku Shoujo (Mitsuganae) has much better soundtrack including awesome ending theme. Openings for me personally doesn't work at all. Japanese voice acting keeps up to its standard. And please don't forget that Enma Ai has no hope so everything she says has to be deadly monotone. Also the repeating of same lines over and over again has its place in story.
Characters are intense even if they appear in single episode. Each episode is based on displaying fate of various people. There are many various archetypes. The main characters are also quite interesting and their nature is slowly revealed as story progress.
Jigoku Shoujo is one of the greatest anime ever been made. It might not work for you, if you don't see the bigger picture in terms of philosophy and religion as story is tend to be repetitive and each episode has distinct template feel. I think Jigoku Shojou is not about horror nor crime and punishment, but rather the nature of evil and hell itself.