Finally watched this yesterday. Rosamund Pike was hilarious as Elspeth, although she was really awful.
Ollie was a monster, but we've all met someone who was a liar or a fantasist or a leech or just a freeloader, so we recognise parts of him. There were moments when you almost felt sorry for him early on, too.
I also like how Emerald Fennell resisted the urge to resolve every single part of the story. There's still bits of the plot to think about afterwards: where is Farleigh now and how did he react? What about the butler? Did Ollie kill James or Poor Dear Pamela as well?