Why a lot of people like Kuroshitsuji (with me probably over analyzing some parts):
It's quite dark, and unexpectedly so. Why? Because of the silly first chapters. But as the story progresses, the adventures of Ciel and Sebastian take on a darker theme. It is not simply about revenge and corruption. It's also about the fallacies of mankind, their hope in the face of desperation, and the will to sacrifice everything for one goal. It's tragic because Ciel is young; younger than most of us who read the manga. Yet he has suffered more than we could ever believe. Is he a strong character? Perhaps not. But as a human being, he perseveres to achieve his one goal of revenge. Will he be able to fulfill it? We have yet to know. It is interesting to see his wavering humanity and how he tries to keep his sanity during his everyday life. In recent chapters, he has become a more mysterious character, especially in regards to his contract with his butler, Sebastian. Ciel seeks salvation by, ironically, giving himself over to evil, thus portraying the weaknesses and the desperation of human beings.
The character of Sebastian is incredibly demon-like. He is cruel, sadistic, and yet loyal (but only to his aesthetics). He is skilled at everything yet he can only do it alone; He is not human and therefore cannot interact with them. He may be the image of perfection but he is not. As we read the story, Sebastian and the rest of the characters seem more and more separated. This disparity, though I do not know if it is intended or not, isolates Sebastian and marks him as a devil. Yet, he can blend in to the society as if he were a human. This is frightening as it shows that evil exists among good, but it is so well hidden that we could never recognize it, even if it should be obvious. Yes, Sebastian is quite a "cardboard cut-out" but I believe that it's only his facade so he can hide his true self. Personally, I find that Sebastian is an interesting character because I just can't quite understand him at times, thus causing me to come to the conclusion that Yana Toboso successfully created a being that cannot be comprehended by humans because he is a demon.
The relationship between Sebastian and Ciel seems to be one of absolute trust. They are so intricately intertwined that it is a bit scary to think how much Ciel has associated with evil. Yet, he does it because he is willing to sacrifice his soul for revenge. Ciel and Sebastian are alike in a way, but Yana Toboso also manages to keep them separated. Ciel succumbs to evil and is more susceptible to it, even without Sebastian's help. His fate lies with the devil, and is forever connected to the dark. Human nature is more demonic than a demon.
The main plot is usually shrouded in mystery and is seldom shown. It is perhaps because of this that people love Kuroshitsuji so much; whenever the main conflict surfaces, the readers get drawn into the darker side of the story... Although, at first, the story seems to be going nowhere, each arc has a subtle foreboding undercurrent that usually keeps us reading. Not to mention the exceptionable art that appeals to the eye and is usually very clean and very clear. Yana Toboso has great designs and seems to put a lot of effort into researching about Victorian England (although, she does tend to slip up quite a bit...).
Although it seems that the main storyline lies with Ciel's conquest for revenge, I really do believe it is more about Sebastian's perspective on humanity which, in consequence, also gives the readers a different view on the nature of human beings. Every character seems to have a darker side to them, and they all have their own dark secrets. So although some characters may annoy the hell out of you at first, you grow to love them...
One problem I do have with the manga is that sometimes, the "fanservice" is a little over-the-top. You'll see what I mean if you read it. Also, as someone has already mentioned, sometimes, the comic relief is completely unnecessary and overdone.
Oh, and I can't believe I have to clarify this: Kuroshitsuji is NOT YAOI. For goodness' sake people! Although it is labelled as "shounen" its target audience is more on the female side than the male side, therefore, be prepared to have many fangirl comments that will make you confused.
With all that said, Kuroshitsuji is my personal favourite. So, of course, I'm going to rate it 10/10. However, this may vary with other people due to the aforementioned problems above.
If you're into the slightly gothic style, Victorian England and mysteries, you'll probably like Kuroshitsuji. It's not the best, but it you may just become addicted to it 🙂