The first series of Black Butler was a fascinating story, involving some very dark comedy, some downright weird historical anachronisms, and some nice interplay between the characters. Little girl Queen Victoria and crazy angels and a "New London Bridge" being constructed of human souls. It was just a fun watch from start to finish.
The second series is by no means bad, and I did enjoy it, but the introduction of Alois Trancy (which ended up being the whole storyline) was really uncomfortable. It took all the weird molestation undertones that Sebastian-Ciel's relationship had (buried behind a "demon devouring a soul" motif) and brought it really far out into the open. Alois is damaged goods from the ground up, he's literally molested by his adopted father and he's got his own father-figure demon to devour his soul, just like Ciel does. But where Ciel comes off as vulnerable (and desperate to hide it), resigned to relying on Sebastian even with the knowledge that the "devouring" is coming sooner or later, Alois is cruel, and seems to relish his fate, and flies off the handle when his own demon decides he'd rather "devour" someone else.
Overall the second series is good, and if you're a huge fan of the first you won't be sorry you watched it. Some of the bonus OVAs included are also tremendously fun. Just be prepared, there will be some awkward moments.