So I finally gotten to some endings, and here are my opinions.
Sera's ending is by far the best so far, no tragedies and I would love to see Mars in an actual caring relationship. Sera deserves Mars more than anyone, and Mars deserves genuine healing more than anything. Both of them becoming goody two shoes is kind of a pipe dream, but it makes me warm and fuzzy inside. I ship this.
Erin's ending is kinda gay, I won't be surprised if Mars ends up becoming a flaming homosexual that falls for Erin and treats the other girls as side-bitches.
Ellena's ending is truly the most vile. She becomes nothing more than a cock addict, full-blown on destroying herself, her son and the world since it gives her pleasure in doing so. Mars has created a monster, one that he now calls "mother". I would not give such a woman even a shred of control over me, she is far too broken. I'm pretty sure Ellena is going to use Mars to bring about total destruction.
I've yet to experience Cecille's ending, but I imagine it's "the cuck life" for Erin, where she would endlessly humiliate him for pleasure. Honestly, we are meant to dislike Cecille as she is extremely selfish and a simple-minded gremlin, but behind all that she really is not anywhere nearly as broken as Ellena is. I'm pretty sure Cecille's ending only sucks for Erin, since she seems to enjoy being the "queen bitch" around Mars, so a harem path might be doable with Cecille in charge.