I had no idea about this movie, it came out 20 years ago and I found about it only today. Watched it on a whim out of curiosity, since I heard it gained quite the cult classic status. I was not disappointed. The movie holds up really well even after all these years, the atmosphere is creepy as hell and dreadful. And for a good reason: the building we see in the move is an actual abandoned hospital (Danvers State Hospital in Massachusetts, google it: it's pretty creepy). The characters are well build, rounded, believable, and the actors do a fantastic job. The movie is horror, sure, but not many jump scare or cheap thrills here. It's not your regular whodunit either. It is all very psychological, subtle, takes time to fully comprehend it, which is why it took people and critics sometime to appreciate it fully. And it is pretty damn scary too, with a sustained level of tension that is admirable, in any horror movie.
The editing, the lighting, the sound, everything in this movie comes together to tell a truly unsettling story and it works great. To some, it might be too slow, hard to follow perhaps, and just not worth it, but to me the payoff was huge. I highly recommend it. 9.0/10.