Hmm, well i personally thought that this series was pretty awesome! The animation is fantastic, and the plot is suspenseful and thrilling..though sometimes its a bit gruesome/horrorific. Even in the beginning, you are drawn into the first episode which wants to make u continue watching on to see what twisted the main character, btw the ending of the first ep, is sooo 'unique' and twiiiisted, whew....
This series really makes u consider the darker side of humanity, but hmm, the main charater, ciel, desire for revenge/darker side doesn't overshadow some of his nicer and cuter qualities, and his butler, sebastien, is pretty epic and just woow..
I would say that this show has little to no romance at all, so I wouldn't consider it yaoi, though there are some parts that doo look shounen ai.
Also, all the characters are quite appealing, even ciel's servents which have a 'interesting' past and add humour to the plot. The undertaker is just a mystery, he doesn't doo much except to give info to ciel and i guess add humour as well. I personally dont see anything too him too.
So if u like gothic, dark animes with bits of humour its a must watch! Though, the second series didnt match up to the epicness of the first, i was pretty dissapointed/sad at the finale, though it was unexpected. Hope this helps >.<