I love the Black Butler manga, and I was quite excited to see the anime. In short: it left me disappointed. I know, I know... the manga's the manga, and the anime is the anime, but still. It could have been much better.
My overall impression was that the manga is a little darker, a little more twisted... there are some scenes that are (in my mind) completely ruined in the anime by adding unnecessary comedy to them. Now, I do like the mischievous side of Black Butler, too, but farce in the wrong place really ruins the taste.
There's a depth in the characters in the manga that I don't think the anime quite reaches - Ciel is brooding and cynical, sure, but as his story slowly unfolds, the manga hints of an even deeper trauma (and certain kind of madness?) than what comes across in the anime. Sebastian, too, feels at times more scary, and... even Finnian managed to creep me out once in the manga - which he decidedly does not do in the anime.
Still, there are a few excellent episodes too, which make this first part... well, if not quite worth 50 dollars, almost. The series begins quite strongly. (The second part I most definitely won't be buying. I've seen it all, and I couldn't have been more disappointed with it. Three, or maybe three and a half stars for this first part, one star for the rest.)