The New York Times opens with this: "The claws draw blood in “Notes on a Scandal,” a misanthropic game of cat and mouse from which no one emerges unscathed". The Guardian says, "Something so horrible and abject shouldn't be so compulsively watchable, and yet it is." The Independent has some of the same plot-spoiling concerns about the film that I do.
Empire Online calls it "Intelligent, classy and skin-crawling." Rotten Tomatoes has a critics score of 87%.