I'm not yet current, but I've watched the entire series and gotten rather far in the manga, and I've got to say, it's one of my favorite series. Here's a series with depth that's character-based and somewhat poetic. Every chapter is very interesting even if it just amounts to talking, and while some things are definitely stolen from other series (the whole Raimei story arc, with the exception of Raiko's character seems way too much like Sosuke), the brilliant characters more than make up for it. I really enjoy the art style as well.
Nabari no Ou is Shonen-ai. Sort of.
! Except Yoite doesn't really have a gender at all.
If you can get past two sets of boys having relationships closer than friendship (but, I must note, without being romantic), then you have an amazing series ahead of you. I actually find these relationships quite interesting and they add to the series rather than feeling like fan service, which, when you watch this series with a bunch of teen girls, seems like it.
The anime doesn't even really give credence to the "ninjas in modern day" concept, even while sticking close to the manga until the very end (where the change is appropriate considering they don't expect a second season), so if that's scaring you off, don't be. The ninja aspects are more in the manga, and they're more interesting than lame or forced. It never feels like a gimmick.
Great series, this.