The better opinion i read is the one Haily post. That's the reality, Black Cat dowsn't give us too much of a new concept but tons of hellish fun. It's entertaiment, that's what this is. Cowboy Bebop + Trigun + Gungrave + Kenshin but softer in the emotional aproach to the characters.
Train origins are the same as Spike, but we actually get more explanations about his past, Chronos seems to be just like Red Dragon. The difference is: even though Vicious and Creed are just f. crazy, the last one is a complete gay in love with Train, and the Black Cat never likes Creed and Spike was friends with Vicious. Anyway the problem start with a women, yet i love Saya and hate Julia. Kenshin like? Where? He tries not to kill, but unlike Kenshin, Black Cat doesnt have any personality conflicts, his story is not as tragic, and he knows that if he needs to kill he will have to, unlike our beloved Kenshin who sacrifices himself not to kill. Trigun and Gungrave (i guess you know that these two series actually are from the same universe) oh well the Sharpshooting skills of Train are a lot like the main characters in these two series. Yet Train is more simple than the main characters in this titles, and nobody hates him for playing dumb. Train is a charming stupid guy, who is in fact a pretty tough guy trying to enjoy his new life.
Sven is a Bishie version a Jet Black, he was a cop, he lost a body part, he is strong and smart, and even a great sharpshooter. But! He is almost every time left out of the fun because of his reckless partner. Is he just like Jet or not? Though... the Foreseeing Eye and the Glaspar Eye really kick ass.
I wont go on, this will take an entire day... Rinslet = Faye, etc, etc... Or the situations... there are too many common places, yet i love Black cat