Seeing the array of reviews here, I realized most of them are biased, so I'm going to be frank. Instead of going on about how much I love or hate this manga, I'm going to give the things that stuck out the most.
Gorgeous artwork: There is a level of detail to this manga that I don't usually see in of shonen works. You have textures, variations in coloring, facial details on close up... the list goes on.
Humor tinged with dark undertones: Anyone that reads the manga will say that it starts off dark. However, a common tool in writing is to introduce static characters as comic relief. Kuro Shitsuji does this. While the initial chapters are dark, spots of light peak through in the form of Ciel's staff, the infamous butler included.
Seemingly pointless plot arcs: Ah, the curry arc... was there really a point to that? It was hilarious, but it didn't seem to add to the plot. Of course, since the series is not finished, we really have no way of knowing whether or not this arc pertinent. Recall the second season of Ghost in the Shell. Seemingly pointless episodes played a part later on.
Plot isn't immediately obvious/ character motivation: Many mangas introduce the goal of the character in the first five minutes and then spend forty plus volumes having the character complete this goal. Not Kuro Shitsuji (Naruto anyone?). You know Ciel has a goal at the beginning, that much is clear, but the finer details of the goal remain obscure. We know he is hunting someone, and that he was humiliated. The rest of the details are given in snippets throughout the manga. This is good story telling. A good author never reveals all his/her cards at the beginning, ^_~ In addition to this, there is depth to Ciel's goal. It is emotionally driven and passionate in a believable fashion. Even though we don't know what it is, we see how it influences him. To see a counter example, consider the Inuyasha series. The main character, Kagome, has a very simple goal and there is nothing left for the reader to discover. Inuyasha's character is a bit better, but still easy to figure out. Ciel's mind is a maze of idea's that we have weed through to figure out why he behaves the way he does.
Ciel Phantomhive: A flaw in many manga/ anime is the lack of a strong BELIEVABLE main character. Ciel fulfills this role. He is not a sweet little angel, but manipulative and conniving, wanting to do whatever it takes to get is goal, but this creates faults. In addition, he doesn't change. A lot of characters have 'major revelations' and change for the better for it. Not Ciel. He has revelations, but continues on with achieving his goals in the same manner as before.
Sebastian: Oh, the butler, so many references to a Gary Stu. He's perfect at everything. It's exaggerated, however, so it's humorous. In addition, you don't know HIS motivation either, though it is clear that he has a reason for serving Ciel. With Ciel at least you get an idea of why he acts the way he does, not Sebastian. Subtle hints, like well placed glares, shrewd glances and such indicate that there is more to it that the author and artist reveal on the surface.
Shonen-ai hints: There are hints of romance between Ciel and Sebastian. No need for elaboration here, the manga speaks for itself. However, it is possible to ignore this aspect as it is not a huge focus and is only subtly hinted at.
Now that I gave the highlights, it is up to you, dear reader, to interpret them and see if you are interested in reading the manga or not.