This was rather - no, very - impressive; reviews talked about it being over-long and unsurprising but not so, imho.
It was from the start very well and intelligently crafted - the restricted colour palette - much black and white - meant that when one did see colours at key moment - Polly’s hat with the vivid yellow and green feathers in it, the blood on the floor - they had real impact. The movements too, matched by the camera, added to the growing sense of dread and tension. the story was unfolded slowly and powerfully. The use of the heavily ironical song playing in the dying author’s bedroom was particularly effective.
OI loved the little details that were clearly but not obviously pointed out from the start like the turned-up corner of the carpet at the bottom of the stairs, the claw hammer (and verbal references to a claw) and the very creepy backwards turned feet. In other films these would have been major and perhaps sensational dramatic moment but here they were all the more powerful for being so restrained.
So, worth seeing - but give it time and let it work upon you; I found it worthwhile and wholly engaging.