That's my first time writing a review, and considering the fact that my English is far away from perfect, it may not be very enjoyable to read, but I wanted to write it. No spoilers in the review, so you can read it even if you don't know the series.
I've been following Nabari no Ou for more than two years - I liked the anime, so I started reading the manga. There were problems with scanlations - missing chapters and such, but somehow I managed to be up-to-date when chapter 49 was out. Since then, I've been eagerly checking out the new chapters every month. This series really got me.
The story didn't seem too complicated. The main guy has an omnipotent power in him, good guys wants to erase it, bad guys wants to get it for themselves. But soon it's revealed that bad guys aren't that bad, and generally the whole situation isn't so simple. Nevertheless, I can't say there's much of intense action in this manga. I think the most intense arc is Alya arc in volume 6 and 7, and the last chapters. The plot itself is rather slow-paced. I gotta say that I really liked the humour of this series, too. Generally I think this series was more about the characters than about the plot.
Yeah, characters... If you watched the anime and didn't like them, don't get discouraged. The characters keeps changing during the series (character development is something to praise here, especially for the main character), and our view on them keeps changing as we're coming to know more facts about them. Any of them isn't black or white, everyone has their reasons to fight, and sometimes emotions clash with those reasons. Sometimes they act unreasonable, but it doesn't stand out too much, and in some cases that unreasonableness was explained. Most of characters' stories, along with what happens in the series, are really touching. And what is most important - most of characters are likable. I, personally, didn't hate any of them. And it's hard to choose a favourite character for me, but if I'd have to choose one, I guess I'd go with Aizawa.
Besides of characters and their stories, what makes the series more touching is the drawing style. It kept changing too, though. At first it wasn't too good, but it evolved to be pretty awesome. I gotta say that I don't like the author's way to draw characters too much - they were way too skinny for me, and the eyes were too big in case of the younger characters. The designs were nice, characters were nice too look at and easily recognizable - just Rokujo's parents were weird case, because his father was very similar to Koichi, and his mother looked almost exactly like Miharu himself. So, what was awesome about that style? How great it was showing the emotions of characters. The pages without words were saying more than a thousand of words. I actually cried a lot reading this series. Ah, and the backgrounds were very nice.
Overall, I loved this series. I think it's my second favourite after Fullmetal Alchemist - just because the plot itself was better in FMA (but I gotta say that I liked Nabari's end better than FMA's...). It was nice to follow it for those ~2 years. I give it 10 because of the enjoyment.