Well, the story itself was pretty basic. There weren't really many shocking twists or amazing plot ideas going on. However, it was definitely enjoyable, and there wasn't anything wrong with it, per se. I guess for me it was just a classic shounen at its finest.
The art was awesome. It was always nice to look at, and I felt like it slowly changed throughout the series. If I had to say, by the end everything looked a little bit prettier.
I feel like the characters are part of what made this manga so enjoyable for me. I fell in love with Train and his quirky personality and fascinating past. Eve was cute in her shyness and intelligence, and by the end her fighting skills were quite impressive. Sven just fit in well together with the two of them and had his own moments to boot.
Other characters I found interesting were Creed and his obsessiveness with Train, Kyoko in all her hilarity - just doing whatever the hell she wanted - which was sometimes cute and heart-warming, other times really cruel, Woodney just being who he is, and many others as well.
I must say I loved this manga. It is probably my all time favorite manga. I can't honestly say it's the best manga out there, but it's the one that has stuck with me for years, one I'm not likely to forget and always to love. I'd say give it a shot, if you like shounen like Bleach, One Piece, etc. there's no way you won't like this.
If I had to recommend one manga that I feel is similar to Black Cat, it would be Rurouni Kenshin. Both mangas have hopelessly lovable main characters with interesting pasts, who are incredibly strong and fight to protect what's important to them.