since every review seems to be giving the story a summary, I don't think ill need to include another version in mine, so anyway if you're into dark plot, but not at a horror or overly disturbing level (PG-15) then this is for you.
Regarding the characters, Ciel is a walking mess of contradiction - which makes him quite a tortured and interesting character. I wouldn't say he's realistic, but for a manga, I think he's 3 dimensional enough. Sebastian is a bit cardboard cutout, but tolerable (given what he is) and his sarcastic egoistic personality makes up for it. To be honest, their relationship seems to be the main attraction for this manga - at least that's what bringin' all the fangirls to the yard. It's one of those: not shounen-ai, but using that aspect as fanservice and I don't blame the author, it's a pretty natural route to take for that kind of plot and 'half shounen half shoujo' esque kind of art
the other characters do okay as supporting roles, most of them are quite eccentric and rather weird, which gives the manga a good shounen dynamic... the comic relief is nice, but sometimes overdone and unnecessary
the plot is separated into arcs and each is based (sometimes quite loosely) on British historical or folkloric events, which is often a nice spin. The author seems to have the habit of alternating the arcs between dark and humorous one, its up to you to decide whichever is your cup of tea. As for the mystery aspect of the manga, I don't find my jaw dropping at the plot twist - but maybe that's just because im too old for this kind of plot
the art is something you might want to read for. Like I said, it has a half shoujo, half shounen esque style, much like any other mangas published by Gfantasy - the latter chapters seem to lean more towards the latter category, with the increase in action sequences. Oh and the author has a knack for victorian fashion with a modern gothic touch so the clothing is nice to look at. The art is certainly detailed enough, but still stays very crisp and clean (as someone has mentioned), and the composition gives a nice dynamic and impact, which is a another plus
so to sum it up, this manga is popular because the author knows who she's targeting and what's she doing - not to mention, it's marketed by square enix for god sake. So if you're into stuff like Pandora Hearts, Nabari, Code Geas or even KHR, give this a try.