It's difficult to review Bly Manor without thinking of Hill House, which was, in my opinion, far better than its sequel. However, with that being said, Bly Manor is still a solid watch as a stand alone season. It's not a "horror" show in a conventional sense - it's less about ghosts and more about the ghosts of the characters.
Like Hill House, the first four episodes are slow, building up to a climactic episode 5 and after that, there's no going back. What makes this show work is its characters - you continue watching for them, specifically Hannah, Jamie, and Owen (who are, unfortunately, not even the main characters). There are phenomenal dialogues/monologues surrounding love, death, and everything in between which are insightful and heart wrenching.
I would probably have given it four stars if it weren't for the flashback episode. That was too slow and boring, and seemed like filler. They could have developed the story of the Manor in a more dramatic/interesting way, instead of retelling it like a history lesson. More showing, less telling. Also, the British accents of the American actors were definitely hit or miss.
Still, I kept watching because the story began to unravel after the 5th episode and i NEEDED to know how it was going to end. Once again, the last episode dragged on for me, but still tear-jerking. Overall a good show for a little spook and unique storytelling, but if I had to choose, it's Hill House all the way.