First things first, to my knowledge, this manga is NOT finished, nor is there any estimate to when it will BE finished. There was an extraordinarily long hiatus, and then there was vol 12, and now...no clue.
Moving on.
Story: 8
The story follows Asato Tsuzuki and his partner, Hisoka Kurozaki, on their investigations as a part of Enmacho as Shinigami. Each investigation is very different, though, there is often a recurring antagonist, Kazutaka Muraki. As the manga progresses, things start to become more interwoven, and more information about characters comes to light. Now, I give Story an 8 due to the fact that a lot of things seem unfinished, though, it seems like they should be at this point in the manga. Perhaps it will be later, and I will have to amend myself, but at this point, I'm not particularly happy with it.
Art: 6
The 6 is totally a personal thing. Yami no Matsuei was started in the 90s, so, it has the art from the 90s. I prefer more modern art styles. I found a couple chapters online that were completely revamped, though, so, the art does change, eventually. After some searching, it appears that it occurred after the hiatus, so, that makes sense. Still leaves it at a 6 for me.
Character: 9
I ADORE the characters in this series. There is so much to them. HOWEVER, the incompleteness of some things that I mentioned in Story comes into play here as well. Particularly with the main character. That is the only reason this is a 9 rather than a 10. All the characters are so much fun and have so much depth when put in certain situations. I love it.
Enjoyment: 10
For all the faults, I love this manga. I sped through it in...three days? Ish. I'm very disappointed that it is not over, as there is a lot going on right now, and I NEED TO KNOW!! Ugh. This manga seems built around dropping little crumbs here and there about the overall story to keep you hooked, and then throwing in this left hook arc that you just get absorbed into.
Overall: 8
It has its faults, it's incomplete, it drives me nuts, but it's GOOD. So, 8 seems about right for it. I really hope this manga isn't going to be dropped. One page said that the author had a lot she wanted to do with her story, and she was excited to keep going as long as sales kept up. I hope she still feels that way now.
Note: This could fix all my issues with the series, but, I'm broke. I read this online. All of it. As is quite well known, translations don't always happen the best. Things get REALLY lost. So, keep that in mind as you decide if this is worth picking up.