This may be the most well written anime series I've seen. Not that it was Oscar-worthy writing, but most of the writing I've seen in anime has always been the thing that prevented me from really loving it. This series doesn't have the lazy, shortcut type of writing that simply fills the plot gaps. It actually builds different stories to support different themes.
Anyways, the series themes of revenge and redemption are woven together well with well-crafted story lines. This isn't an anime with non-stop action, but it does have its share of dark supernatural battle scenes, as well as witty humorous battle scenes. There is quite a bit dark humor in different parts of the series, but it's balanced out by traditional anime slapstick comedy (which I'm actually not a fan of and part of the reason it didn't get 10 stars).
If you watch the dubbed version, the translation and acting are all excellent. Some hardcore anime fans complain about the fake cockney accents, but they're mostly limited to some supporting characters. The voices for the two main characters (Ciel and Sebastian) are portrayed terrifically and are true in tone to their Japanese voice counterparts (I saw season 2 in the subbed version). Simply put, this may be the best job of versioning I've ever seen in anime. In its early U.S. import days, anime had horrible translations in both subbed and dubbed versions, due to what I expect as literal translations of the script from Japanese to English. Funimation has really done well in this department, as they've taken the meanings of the original Japanese script and rewritten it in English with English idioms and phrases that work perfectly with the characters and themes, rather than just lazily taking the Japanese words and translating them directly into English words, creating phrases that are unusual or that simply don't make any sense.