Really good, some flaws (SPOILERS)
I enjoyed the film a lot. It was creepy, didn't rely on jump-scares (alone) and had a good relationship between the two main characters - both quite well-performed as both adults and children.
The design of the film is good, too, and I particularly enjoyed the mirror's look. It's got a good feel to it.
One of the best things about it is that the exact nature of the monster isn't really clearly known. It seems to want to kill and possess souls, but outside of that we don't know what kind of entity it is. That and the insanity-attack it seems to bring against its prey gives the film a very Lovecraftian vibe at times.
Complaints:
1. This one is minor, but I'd have liked a bit more "is this all just insanity" before the supernatural elements came in. Tease us more with the possibility that this is just a family going nuts and not a demon (or whatever)
2. I know this is contrary to not knowing the nature of the creature, but I'd have liked a couple more defined rules about the mirror. It was so open-ended as to what it could do. It could mess with phones, yet Kaylee used her phone to "prove" reality and unreality. It had a sphere of influence, but it was a moot point because it could make you hallucinate yourself running away. A bit more clarity on its powers (not its nature) would have been welcome.
EDIT:
When I first wrote this post, I included a bit about how I wasn't 100% satisfied by the ending, wanting it to end with the timelines merging more, messing about with time, having the child and adult versions of the characters help each other, and/or reveal that the adult timeline was pure projection.
I thought about it for a moment, and the problem with all that is (in addition to layering in a sci-fi time-twist element far too late in the film) that this was what I was thinking was going to happen during the movie. It's too predictable.
Although the ending as-is isn't outside the box for this kind of malignant evil horror movie, it wasn't what I was predicting would happen, it felt like a great conclusion to the story(ies) and the fact that the two timelines mirror each other seems relevant to the story somehow... Something about reflections...?
Anyway, I liked the movie a lot and I think it deserves more acclaim than it seems to have acquired.