Just finished the third season, I don't know what it is about the Jigoku Shoujo series but I can't get enough of it (I think it's Enma Ai's charm when onscreen, lol). They seem to have a pretty decent formula i.e. First 10 or so episodes are pretty hohum, then in the middle you get a hint of some overall plot then they just seem to explode with story content in the last 2 episodes.
I didn't think they could make a Season 3 after what happen in Season 2 but whatever bring on more Jigoku Shoujo (Season 4 please)!
Edit: Fixed misspelling of Yuzuki's name as "Yuki".