To sum it up, Kekkaishi is about a teenager named Yoshimori Sumimura and his childhood friend and love-interest Tokine Yukimura. They go to their joint junior high-high school at night to defend it against demon-like things given the name Ayakashi. They do this night to night while still maintaining the identities of regular students during the day.
I'll admit it, at first, I also didn't think much of the show. I thought it was just another anime like Bleach where the main characters just fought every episode. Boy was I ever wrong.
After watching the show on adult swim, I started taking a liking to it (mere exposure effect at its prime). And as I started to like it more and more, I wanted to find out more. So I started to google it and look up the manga. And man, it was worth it.
In regards to the anime, which has its own particular changes to the manga, it was well done. At times, I liked the anime's way of progression slightly more. While I'm still waiting to see the conclusion on adult swim, the characters are well balance. Yoshimori's comedic flare adds levity to the story which is juxtaposed with Tokine's seriousness. Yoshimori's MO is a bit cliche for an anime, but what isn't as cliche this time round is the involvement of the characters. As I watch a few animes here and there, most of the shows I've seen become protagonist-central. The whole thing slowly turns into the main character pulling something out of their butts to win and win and win. It's different in Kekkaishi. Sure, Yoshimori still does some crazy stuff to win, but Tokine still has an active role in battles and shows growth. The other character, Gen, also manages to grow and fight an 'equal' amount battles. Even the two grandparents play their supporting roles rather well. This was an eye-catcher for me. The backstories of the characters also drew my attention.
Basically, it was Yoshimori was an easily frightened child who didn't like being the bearer of the houin mark and cried a lot. He was close friends with Tokine, despite their family ties. I found this interesting because, well, usually it's the other way around so seeing it flipped was pretty fun.
In the end, I would definitely recommend watching the anime and reading the manga (as you wouldn't want to leave the story half done, would you ;) ). It's definitely an exciting story, full of twists and turns while keeping things fresh.