It was a pretty good movie! It was properly creepy. The pace was pretty slow at points; it did dedicate a lot of scenes to people just gawking at the entrance to the backrooms, but I can appreciate a slow pace, the vibes were good. The movie really did have a good soundtrack. It also had quite a few really good scares. (I rather liked the one where the monster picks up the camera after Clark sets it down.)
I didn't really expect the movie to end how it did, but I personally didn't mind the ambiguity and abruptness. The movie got across what it needed to, and didn't really see a need to resolve Mary's fate (and I personally agree with it on that front.) I found the movie to be a pretty obvious metaphor for alzheimer's, with it going on about how the Backrooms remembers things but "wrong," I never really considered the other looks people had on the movie, like interpreting it as a metaphor for AI or various other things. (The internet is neat, I get to hear so many new opinions I never would've heard otherwise!)
I know some people had issues with the characters. I didn't really notice any issues with the characters, they seem well developed to me, but I'm a bit of a numbskull so idk.