Even though this is a spoiler thread - warning about spoilers below.
My wife wanted to see it because she read the book. I went into it cold knowing nothing.
It was meh. Pretty clear she was faking the whole time and the twist would be on the husband.
All that said - Verity could've come clean when the girl confronted her or any number of times. Worst case, the husband read the autobiography and believed it so I don't see him as the bad guy. Or Verity was a psycho and did kill the kid and the dude wasn't the bad guy at all.
I lean towards her being psycho but it's clearly meant to be open. The autobiography was supposedly written to be the opposite of everything but the teeth marks she talked about were actually in the bed. Also why would she have told the other kid not to talk about what happened the day the sister died if she just drowned.
It was clearly an oversexed female targeted movie which I'm guessing the book was too. My wife who read the book didn't like the movie but did like the book even though it was a long time ago she read it.
The most interesting part to me was thinking about reading some books from the villains POV like Verity's stuff was.