I watched this on a plane a few months ago, and immediately gave myself a massive headache trying not to cry in the middle of a plane.
It's an epic tearjerker for sure. I see why some critics' have panned it; for sure the plot is a little contrived, at times a bit creepy and does have a few holes in it (wouldn't the mother just take on the kidney her daughter now has no need for?) but if you look past that, it's brilliantly written, acted and shot.
My one criticism is that it could have done with being half an hour shorter. I came into it not aware it was a Christian movie, but that does make it accessible to younger, or at least teenage, audiences.
The script and acting on its own draw you in though, and it doesn't rely on overproduction or sensationalism to stand up.