I would agree that Hell Girl can be repetitive, but it's still interesting. Without giving too much away, a subplot's developed in the story with a reporter investigating Hell Girl and his young daughter somehow being able to sense when Ai and her henchmen are about to whack someone, leading him to try and stop the people seeking revenge from carrying it out, but he comes up short. That's livened up the story considerably I think.
I wanted to watch the second season, but after a few episodes I got bored, probably its only me, but after what happened in the first season, the beginning of the second seemed as if they started all over again.