A lot of folks commenting here have spoken about the realism of the characters' motivations, so I won't speak about that. I too was expecting a manga like Shaman King, and was stunned with the contents of the first chapter.
In the past three years, I've had a number of intense spiritual experiences. Some them relate to past life experiences of one kind or another. Based on my own experiences, this manga depicts this in a very realistic way. I say this because I often meet folks who tell me that past lives seem to be a fantasy, and I tell them that, often times, they are related to things about your journey you don't want to see. There are good times, and there are a lot of really intense times. Sometimes, meeting folks harking from a past life is brief; sometimes it is a bit longer. Having access to past lives have more to do with healing the wounds you didn't even know you carry, than it is as a way to tell people how special you are.
The story is depicted well enough that I was moved to tears many times over the course of reading the story. Not so much for the tragedy, but rather, for the sheer beauty of how souls come together, the kind of cosmic orchestration I have seen in my experiences. The healing that takes place, it's all done out of love, both the personal kind and the transcendent kind, and I can clearly feel that from the story. And it makes me wonder... what has this mangaka experienced himself?
For the record, there are a few details that are there for dramatic or for storytelling purposes.
The manga does a good job of showing how people move from culture to culture often and changes roles and relationships. But the kinds of lives one lives varies a lot more. You might change professions or utilize vastly different talents. There are also changes in gender ... but that probably would have complicated this story.
People don't look the same either. However, when you recognize someone from a past life, you get taken by a strong sense of familiarity. Sometimes, the facial features you see flashes into one you might recognize. I think the mangaka, in choosing to emphasize certain features of each of the character, does a good job relating this aspect of it.
I've never seen a spirit circle. That's not to say that doesn't happen, it just hasn't manifested for me and I had not heard something like that manifesting for other folks. It makes for a good plot device though.
There is no such thing as an "old" soul. Every soul has been traveling the cosmos for a long time. Though it is true that some souls are more aware of others, and it has more to do with the amount of awareness one practices one lifetime after the next.
The birthmarks, and things like having your hands and arms chopped off doesn't always manifest so cleanly. There are other ways past life trauma can manifest: diseases, ailments, allergies, near-sightedness, etc.
I'm glad this manga got written and published. Perhaps, it might spark a few readers around the world to investigate this and heal.