NOTE: Some possible spoilers in the post below, do not read if you have not yet seen the show.
Black Cat was guilty fun for me. did it blow me away or dazzle and amaze, no. but it was fun and so i liked it

the manga is worth looking at from the first 6 or 7 GNs i read. but it could also be that Black Cat just isn't for you which is OK too.
Emma rulz. know this
Black cat was definitely entertaining for what it was, so it isn't as if I have any problems with it, I just wanted to care more about the characters than I did. I just felt like each character was a bit un(der)developed. I really enjoyed the first two discs of the series which set things up nicely for what could have followed, but by the last two discs it felt like they were just throwing it all away.
*POSSIBLE SPOILERS BEGIN*
For instance, Saki (sp?), the girl he meets in the mountains...what's up with that 1 episode arc? It ended up being a filler episode when I thought they were going to reveal some important story elements. Did his defeating the attackers magically make her problems go away? Was there some deeper connection between Saki and Saya? Did I miss several minutes of dialogue that explained all this? I felt like I did.
And then, they add in something like a half-dozen sweepers to fight Creed and I'm suddenly supposed to care about these guys? What should have been rather epic ends up being a bit anticlimactic.
And finally, after Eve is taken we see Train all gung-ho about getting her back and I wanted to believe that was okay, except for the fact that to this point Train has shown only mild interest in the girl, seeing, maybe 2 or 3 real "moments" between the characters.
Before I forget there is also the whole deal with the guy who trained him (name escapes me) in his last battle before saving Eve. I wanted to feel more in that battle, but considering we were only introduced to that part of his past a few episodes ago, and then completely avoided for the following episodes, well, I couldn't muster up any great amount of feeling.
*END SPOILERS*
Thinking about it, maybe the dub performance (which is what I watched) just didn't bring out the proper emotions it was meant to (I didn't watch it subbed so perhaps the Japanese audio is better in that respect.) That said, I've liked all those actors in other parts, so that might not be it either. Like I said, at the end of the day, I just couldn't care as much as I wanted to, which is unfortunate because I love to become emotionally invested in the characters.
Oh well, I don't feel like it wasn't worth watching as I was entertained for the time it took to watch it. I just don't think I could really recommend it to anyone else. Unless I was speaking only of the first two discs. Those two first discs were great.