For me, this is one of, if not the best, ongoing shounen series. Sure, you get the old shounen themes of fighting demons (aka ayakashi), protecting friends/family etc, but Kekkaishi goes far beyond that. What makes it stand out in such a crowded genre is the sheer imaginativeness. Tanabe-sensei has devised a fighting system which is complex and well-considered. It is original yet simple to understand. It's not just a tired rehash of spell-casting/summoning and you can tell Tanabe-sensei has put a lot of thought into how she can make the most of the fighting system she has invented.
Another reason why Kekkaishi is head and shoulders above most other ongoing shounen is that there is an undercurrent of subtle politics. Power struggles and issues of trust are portrayed so well that younger readers should have little difficulty understanding, but adults can also enjoy it too.
Since it is a shounen series, it is inevitable that cliches will crop up. Kekkaishi does not escape them. One of the most disappointing aspects is the Gen story (about volume 9 or 10) where the series is reduced to a typical shounen friendship-or-power storyline.
However, quibbles aside, the pros about Kekkaishi far outweigh the cons. The characters are all likeable (the main girl, Tokine is not an annoying whiner) and you feel like the story moves along with every volume rather than relying on episodes or filler.
All in all, it is most definitely worth a read, even if you're not a shounen fan.