Kuroshitsuji is by far one of the best mangas in existence. It made a huge impression on me, and I definitely think it's something everyone should at least give a shot.
Plot
It's a wonderfully unique story about a young boy in Victorian England who, after being kidnapped, tortured, and sacrificed, makes a deal with a demon: his soul in exchange for the demon's help with avenging himself. The main storyline takes place 2-3 years after the deal was made, and as the story progresses through the different arcs, you get little forshadowings (i know, not a word) of what's to come.
The greatest part, when reading this dark intense story, is that in the back of your mind, you know eventually that what it's going to come down to is Ciel (the boy) finally getting his revenge (we still don't know who kidnapped him, by the way) and Sebastian (the demon) finally taking his soul.
Or not?
As you read this story, if you are like many fangirls of Kuroshitsuji, you'll find yourself hoping for the alternative ending: Sebastian deciding not to take Ciel's soul. So far, there's no evidence pointing towards that, but it's still possible, and the fact that none of us can be sure of how it will end is part of what makes this manga so intriguing. And just for the record, I don't usually read a manga unless it has a romance in it. I just don't. But this manga doesn't really have a romance in it (unless, arguably, you count the relationship Ciel and Sebastian have-we're not exactly sure what that is, though) and still it is the best manga I have ever read, just because it's so interesting (the plot, the concept, the characters). And if you don't like suggested male relationships, you'll still be able to enjoy this story. Because their relationship can be seen in a whole lot of ways, and there is no proof of romance.
Characters
The main characters are very interesting. Ciel, because you just can't figure out if he's a good person or a bad person. He does a lot of "bad' things, and he won't hesitate to kill someone he thinks deserves it, but at times he'll also show some kind of compassion. You get the feeling that he's the sort of person who's willing to make the tough decisions and be the bad guy, even though it hurts him, just because someone has to. You also get the feeling that, while he shows a very cold adult exterior self, on the inside he's very human.
Sebastian, on the other hand, is just the opposite. On the outside he appears friendly, even warm, but inside he views human beings as stupid, and he's unable to see the good in anyone (save for, arguably, Ciel and Agni-this guy who shows up later). Everyone loves Sebastian. Which is funny, because if he met any one of us readers, he'd probably laugh in our faces. But he's a very awesome character. And he's also interesting, because you're not sure what he thinks of Ciel, other than that he is amused by him. Their relationship is great.
Anime
What I've got to say about this is: watch it. About a third of the way through the first season, it starts to stray from the manga, but that's fine. It develops it's own very well-thought out plot that is very realistic based on the characters. And Sebastian's relationship with Ciel becomes so interesting in the second season. He becomes very possessive of him, even getting jealous at times (and believe me, there's almost nothing better than a jealous Sebastian) and he seems to start to view Ciel as a person rather than just food.
The anime and the manga both offer interesting storylines, and while you should absolutely read the manga, no question, you should also definitely watch the anime. They do a good job of making the episodes fairly light, but making the overall tone extremely dark. There are some really emotional moments, especially towards the end of the first season and around the 9th episode in the second season, which is good.
Art
The art is some of the best, if not the very best, art that I've ever seen in a manga. The author has a very sophisticated clean drawing style that looks very professional.
In conclusion...
If you like dark humor, this is for you. And even if you don't, this is for you. Trust me. It's about the furthest thing away from the manga I normally read, but I loved it. The fact that the storyline is so unique is what makes this manga so special. And god, sometimes you get so envious of the closeness between Ciel and Sebastian, and you just want to get inside their heads to see what's going on in there. But while you can guess as to what they're thinking, you can never really be positive as to what kind of people they are, or why they do the things they do. But you love them anyway, and so you don't really care whether they are actually good or actually evil. Because there's so much more to it than that.
By the way, it's listed as shounen, but that's not very accurate. I think it's put in that category because it's definitely not a shoujo, but I feel like this manga sort of deserves a genre of it's own, because it's not really a shoujo or a shounen.