WARNING possible spoilers WARNING
So the plot starts out with a child who's parents are dead, and his butler. The child, Ciel, is a noblemen who now runs the toy company and is the guard dog for the queen, helping keep the illegal underground of England calm and helping solve murders that deal with possible black magic. The butler, Sebastian, is a demon who was summoned by Ciel and now under contract works for Ciel until the murderers of Ciel's parents are dead. The point of Ciel's adventures is to use his place as the queen's guard dog to find the killers of his parents, however Ciel is well aware that when this mission is accomplished, Sebastian will devour his soul.
So is the plot good? It isn't anything to write home about by any means but it isn't badly written either. There are a few twists to the plot towards the end, and we do get bits and pieces here and there to the plot in general. While the plot seems like it will be like a Sherlock type mystery, it is more about creatures of the night and things between heaven and hell which the plot works out well. No real flaws.
Sebastian and Ciel are awesome characters. No one else makes any sense. Grell outside of his original appearance is pointless and more of a comedic relief character that will fall flat to most, and the other servants of Ciel's house have their moments of awesome and humor, but also cna be more annoying then they are worth and their accents often times will switch and make little sense in context.
This being said, none of the characters are exactly bad characters. Grell has his moments where he isn't completely pointless. Also a point is the main antagonist of the series is complex and insane, but well written with insanity so it works out well. Most of the characters at times are pointless and have eye rolling jokes that will at times make you laugh, and at times will make you groan. It is hit and miss with most of the characters, the only good ones through the entire series being Sebastian and Ciel.
Humor is not the best thing for this anime. Sometimes the humor works, mostly because Sebastian doesn't react to most of the insanity roaming the halls around him, at least it looks like he isn't reacting. People's reactions to him are particularly humorous, but without him the humor often times won't work with everyone. Still, the more emotional message of Ciel and living in hatred is well done, and the ending is done incredibly well emotionally.
With the more emotional areas being well done, and the action being fairly decent though few and far between, it is the humor that gets in the way as the humor is funny at times and a miss at times as well. For the most part with Sebastian in tow the humor does rather well as stated before, it is when other characters are the main focus that issues arise. The emotional pull is mostly acceptable with only a few blemishes in humor.
Hardly a dull moment in the episodes honestly. The beginning likes to move along rather quickly but more towards the end the pacing is rather slower which works. The quickness of the anime hurts an arc a bit but it isn't bad, just a little quick on things that should build. Other then that every episode is important to the overall plot of the series without leaving too many questions. The animation style of each episode is also done rather well.
Some of the creativity is in the story line and the ending, and some of it went into the powers and abilities of characters as well as their back story. What bothers me is the inability to rely on something other then convenient coincidence. The plot itself is great but the creativity to get to one plot point to the next, the details in between are left up to the dogs. Outside of that, the settings and the character designs are a little one note much like the characters themselves, and some of the lore the anime creates involving the shinigami or death gods is a little hard to swallow as well as it seems like it isn't structured in the best avenue.
Not trying to say the creativity is bad, but there is a large part of it that relies heavily on coincidence and a large part of it that is unstructured. It gets the job done and does show the viewers an interesting new take on the world of older Britain, but for the most part it just doesn't structure the creativity all that well, as good as it is, perhaps too many ideas to fit in an anime that is 24 episodes? Perhaps the manga doesn't go as quickly. Either way, the creativity is actually really good for the most part, just not structured as well as it could be.
Black Butler isn't that bad of an anime, but the fans are rather rabid about it. For what it is worth, it isn't a bad anime with some more emotional aspects, a good plot, a few great characters and excellent episode quality, but it also suffers from lack luster humor, characters that are only good once or twice and fall flat after, and too much creativity and not enough structure.
The anime is good and I do recommend seeing it, but I'd watch the anime series before actually purchasing the dvds or blu ray. If you really love it after watching it then go for it, spend more money, but if not, then don't worry about it. It is an anime worth taking a look at as long as you aren't incredibly squirmy towards possible Yaoi situations, but I wouldn't outright buy the anime until you are certain you are going to love it. It is good, but not exactly up there with the greats.