I consider saying anything else about it a spoiler, so here we go.
* spoilers ahead *
The piece tackles grief, abuse and mourning in an organic way. Yuta wants to preserve a twisted version of his mother, a lie so he can preserve the good and ignore the bad. This repeats itself with the Eri movie, motivationally we want to leave a positive legacy and these lies we tell ourselves or others are our ideal image. Right before the end the motivation could be that Eri wants to be remembered fondly, but the final twist, if we are to believe it, plays on another idea present in other Fujimoto works. The belief in our lies changes us, and so when she spins the next cycle of her immortal life, she watches this movie, showing an idealized version of herself, and becomes it. Eri has used the film to become a better person, afterall the film Eri loved Yuta and wasn't selfish and flawed. This new person is kind enough to say the film wasn't made for her own gain, but to never forget Yuta.
Other big draw in the material is the open interpretation itself. At what point does the fiction stop? Is Eri a vampire? Was she even sick? Did Yuta commit suicide? Did he form a family or is that last scene acted by his father? We as an audience can't tell the difference, and we can build different stories with the same material.
The artwork tries to emulate the sensations of film in drawing, and as such it could look too contemplative for the likes of manga, but definitely in tone with the piece. Having multiple panels with little to no change is emulating a scene, forcing the reader to mull over the moment and take it in.
