It's something new. It's City Hunter seen from different perspectives. You will view the character from completely different angles than you ever do. [And goddamn, Steel Ball Run totally rocks, completely better than the failure that's part 6. It's not a pseudo-sequel, it's canon, happening in a different dimension, due to Father Pucci messing with time and space]
I mean, this is like just what happened 15 years or something after the end of City Hunter, with a few changes of settings, but if you love City Hunter characters, this is a perfect series to know more about them from the deepest part of their hearts. And it will make you love City Hunter even more. So much more. The situations may be pseudo, because killing off a major character, if canon, may provoke anger from hardcore fans, but the development of characters put in such hypothetical situation are real and canon. Not out of character at all. Just a deeper view into their inner world.
Kaori still hits Ryo with that hammer to keep his pervertedness in check, but she is in a completely different level of maturity and benevolence, so much she is like a goddess, and you almost can't recognize that boyish, pushy girl in City Hunter days.
Every character matured greatly.
Umibozu is still that clumsy hulk but this time he is the one to call Ryo an idiot.
Ryo stays basically the same, with sweet words and gestures that can fell down any woman, any woman I say, but he gives that fatherly, gentle, understanding and loving smile very often. His mokkori is more glorious and useful than ever with clever application in various situations. He is a bigger idiot than he already is in CH, and totally dominated by Kaori, yet at the same time more terrifying mature and knowledgeable when he's serious.
And then we venture in the darkness of Saeko's heart, a far more melancholic, desperate, twisted, complicated, sensitive, self-conflicted character.
There are some really interesting, new regular characters like Old man Chen, Boss Lee, Pantheress Yang Fang-Yu, the little girl that often read fairy tales for Umibozu, etc.
The two young protagonists Shannin (Xiang Jing) and Shin Hon are solid, moving characters with twisted background too.
The circumstances of how the characters met, and the death of Kaori's brother are completely different.
I personally think Angel Heart is better than City Hunter, seeing how Hojo's art and story-telling technique has massively improved. It has a larger cast of interesting characters as well as a larger scale of meanings and themes than City Hunter. It's focus on non-bilogical bonds, family themes, loneliness, revenge and forgiveness, personal justice, attachment to the city, care for children, tragic crimes, and delicate movement of the heart (psychology) etc. , rather than just about love and how Ryo breaks from this girl's to that girls' heart. It has better plot than City Hunter as it's less repetitive and redundant, with similar structure (except for the first 40 chapters) but with far more special situations, regarded both are pretty weak in term of plot and scale compared to huge, top-tier series like Nausicaa, Lone Wolf, or Akira, but that's not what you enjoy it for. If you love City Hunter, you will love Angel Heart. If you for some reasons hate City Hunter, you can't hate Angel Heart for the same reasons.
Quoting my own:
Angel Heart is easily my personal most favorite manga of all time, simply because of the massive emotional impact and sense of warmth and sweetness it provided, which can be found even in its excellent character-driven comedy. It's like Nami's past about Bellemere, but much more sadder, sweeter, stronger, deeper, realer, more mature and delicate, and it has hundred of chapters like that. Like, almost every chapter is heart-wrenching and everyone can easily relate to its characters since Angel Heart thoroughly depict normal city life with all its good and bad, delight and melancholy hidden under the tall buildings and street lights.
Just my two cents.
–- Update From New Post Merge ---
One thing obvious, you will miss a lot of hot girls if you don't read it:
Yang fang-Yu rules.
Ryo is a fool. I would bang her if she dragged me into a love hotel like that.
Are you sure you don't want to see this: