I am very confused which I think was entirely the point. I also want to immediately start a re-read to get a different perspective now that I don't have to try to figure out the resolution.
This is definitely horror, though not the kind full of jumpscares that makes you look over your shoulder nervously when you read. It's creepy, it's claustrophobic and it is scary, but the horror comes from the feeling of unease and the engagement with disturbing topics. It is also surprisingly funny and I now have to read more mid-century American fiction where witty characters just talk to each other because the conversations were a delight.
I liked how the novel played with expectations of horror storys and used that to its full extent. It plays around with concepts of realism and expectable behaviour and while that made me constantly try to figure out what was actually going on, what was the twist, what was the explanation for all the weirdness, it also opened up possibilities for discussions of the main topics: home, belonging, paranoia, reality and imagination, guilt.
It's not exactly a satisfying read where everything comes together, you have nice character growth and a complete resolution, but that's kinda the point. A lot of whats happening isn't being told and the reader is supposed to scramble at the bits of information provided. The kind of unreliable narration that we now think of as overdone but incredibly well done and especially for its time innovative.
Another nice surprise were the queer subtext and the feminist undertones!

Haven't seen the movies but watched the TV series recently and thought, "oh, hey, this would probably be a fun read." Goes without saying that the source material (this book) differs in a lot of ways from recent adaptations. This book holds up surprisingly well, 60+ years later. I had no idea it was this fast paced though. I was genuinely interested in each character's back story (aside from Mrs Montague, she was annoying).
There are some things that don't age well, and they could be argued to be sanism, but I can also acknowledge the age of this book. Mental health care has come a long way. if you can look past that, I think this is worth reading. I need to read more older books. 

A disorienting, sad and occasionally chilling read. On technical merits alone I'm not sure this constitutes a five star read; but the lingering thougths and feelings left behind are enough for me to consider it one for myself. Would perhaps be careful recommending unless I know I'm talking to someone who shares very similar preferences in media, but I'm very glad I read it.

I should preface this review by making it very clear that I'm not the right audience for horror. I get easily scared and can't stomach that particularly well, and I only decided to read The Haunting of Hill House because I don't believe watching the Netflix series of the same name is going to do my mental health much good (as much as I'd love to; I've heard excellent things about it).
As such, I might also not really be receptive to the impact Jackson's work has had on the horror genre as a whole, though I'm aware of it on a theoretical level.

What struck me most about this book, however, was that I didn't find it particularly scary at all. There were chilling moments and there is one particular scene that I find really beautifully executed and terrifying, but for most of the part what Hill House is, that is a profoundly sad novel.

None of the protagonists are particularly likeable, we only know them for the brief amount of time we spend at Hill House with them, and even then only through Eleanor's lens, and Eleanor Vance, our only access to the events that unfold in Hill House, is nothing if not an unreliable narrator.
It's not hard to believe that even outside of her limited view neither Dr. Montague, Theodora nor Luke are pleasant people to deal with but burdened by her own fragile mental state the lines between what is real and what is Eleanor's perception of reality blur.

I didn't find it difficult to understand why Eleanor grew uncomfortably attached to Theodora so quickly, nor did I struggle to see why she felt about Luke and Dr. Montague the way she did even when I found her thought processes and actions worrisome or repulsive. Eleanor is sad, desperate and deeply lonely and never quite manages to fit in — it's always just a matter of time until she makes a fool of herself, as the book puts it, saying the wrong thing, not reacting the right way, letting her interpretations of what she's being told take her down paths that make it hard to parse whether she was right or unfair to do so.

Her sadness and desperation to belong are a central part of the book. Once she doesn't find this idealized sense of belonging she is chasing it's Hill House instead, 'not sane, […] holding darkness within' to quote the opening paragraph, that catches her instead, wearing her down until… well, something has to give.

Shirley Jackson's prose is lovely and whimsical and suits the not-quite-real setting of Hill House very well, but the pacing often drags and only really picks up in perhaps the last fifth of the book.
Not much happens throughout The Haunting of Hill House, either, and when there is tangible, lasting action it's often things happening to the characters rather than them exerting their agency. 
A lot of these 'flaws' that made reading Hill House feel like a bit of a chore at times, even if I understand that they were purposefully placed there. That kind of makes me believe that the execution might not be quite as tight as it might have needed to be at times.

I understand why this book might feel empty or pretentious to some, but I personally found it a fascinating look into Eleanor's psyche and its deterioration and enjoyed it for that exact reason.
I also, and I understand that to someone who might be looking for a hair-raising read this will be a deterrent, really appreciated that it isn't a book that is going to make me lose sleep for a week after reading it.

and they were roommates???

I'd probably rate this higher after a re-read. It was very atmospheric, which I enjoyed. However, I found it hard to follow at times. I believe that was intentional, and a smart choice on Jackson's part considering the direction she took the story in.
 
It wasn't very scary, more creepy and unsettling. Which is what I was hoping for because I am a soft, anxious marshmallow and need to be careful about the content I consume. If you're not a fan of gore/murder/body horror in spooky books I think this is a good option, but do check out the content warnings.