First off, the best reason to read this manga is if you are at all into classic literature. I'm surprised I haven't seen it brought up yet: Black Butler is at its heart a Faust story with Colin from the Secret Garden as the lead. This made me very happy.
Having established that, my least favourite parts of the manga are always the bits that don't have to do with the main story. (And the occasional fanservice. Seriously. Didn't need!)
The rest of the manga is a cherry-picking of the best images of the late Victorian era, around 1890-1910; while technically it is 1889, nods are made to events that have not happened such as the Titanic.
The highlights include zombies & shinigamis on the Titanic (complete with a James Cameron reference, lol), Sherlock Holmes in the Phantomhive mansion (complete with an appearance by Jeremy Brett), and Jack the Ripper (complete with social commentary on the period yay!). The anime also includes the Hope Diamond.
To bring this back to the subject of fanservice, I need to put to rest the idea of a yaoi relationship between Sebastian and Ciel based on their master/servant relationship which apparently is appealing to some people despite the innate creepiness of the very idea. Sorry, ick. =P
The things that Sebastian does for Ciel - help him dress, etc - are rather period. Read some good classic British literature about upper-class kids. Kids like Ciel don't know how to dress themselves. Part of this is because the higher class you are, the more likely your outfit is something that you are not going to be able to get into by yourself; and the rest is because from birth servants have been there to attend to your every need. Why bother?
Sebastian does read as a creep sometimes. He's a demon who intends to devour Ciel's soul. Yeah. I think he warrants a little creepiness. But he never seems to have a sexual interest in Ciel. The author seems to get some fun out of fanbaiting, however. I'm not even sure why because there's obviously no real shipping going on.
As a side note, it's a very dark manga. Lots of blood (and dismemberment in the case of zombies) and the villains tend to commit crimes against children which can be disturbing. And yet it's also very funny. Sebastian, despite his evilness, manages to be very dry and witty intentionally, while at times he's utterly hilarious without meaning to be.
Ultimately, though, it's Ciel who keeps my interest. He's such a small boy taking on the world. Will the weight break him? Will he go mad, or will he go bad? He's got so much that wasn't his fault; but his reactions are what matters. So far, he's reacting very darkly; yet I have sympathy for him and hope that he grows up a little.
Will Ciel, like Faust, escape the deal? Remains to be seen but I expect he will - if he's not destroyed by his own madness. It also remains to be seen whether Sebastian will go soft and keep Ciel from hurting himself with his revenge quest. Again, I wouldn't be surprised.