Hokenshitsu no Shinigami has a plot like every other shounen out there. The premise is unique, but the execution is, quite frankly, completely average.
The redeeming feature of this manga (and, consequently, the reason why I enjoy it so much) is its characters.
All of the characters are decidedly unique, but, consequently, they are all able to be described with a single adjective. Usually, these things don't work out, but Hokenshitsu somehow manages to execute it just right, to the point where I end up enjoying all of their personalities, even if they are crazy and over-the-top.
Personally, I found most of the disease-demon conflicts a bit lame, and many of them lead to shallow (if any) character development. However, the disease-demons do lead to the fleshing out of said characters, even if they don't really change, and that's really what makes this manga worth it. At the end of these chapters, I usually come out liking the character in question a lot more simply because I now understand their ideals/motivations.
My personal favorite is definitely Fuji's disease-demon arc. I thought he was a typical popular boy who was "too cool for school," but his arc really fleshed out his personality and revealed a lot about him... especially about the fact that he really doesn't care that he's popular, and that he's as lazy as anyone could possibly be.
The only weakness, in terms of the characters, is that the manga mainly focuses on the children, and it completely sidelines Hades and the other adults' development/characters, to the point that I actually feel that Hades is the weakest character in the series. (Also, because Ashitaba never gets possessed by a disease-demon, I also feel that he is the most under-developed of the children.)
Overall, this series is definitely a light-hearted shounen, and should be read as such. It definitely has its flaws, but don't take it too seriously and it just might turn out to be an enjoyable enough read.
(Also, I totally do not agree with the comments below me. As I mentioned, I find the characters fascinating, if not relatable, and I enjoy the mangaka's characterization. Mimasaka also just so happens to be my favorite character in this thing. So.)