Before I Go To Sleep had a few decent set pieces and a couple of excellent bits of suspense, but that isn't enough to make a good book. I've given it two stars for these small glimmers of decent writing, because there really isn't much else to recommend. It's on the cusp of a 1.
The dialogue is terrible. None of it is believable and towards the end of the book, with time running out, Watson commits the cardinal sin in my eyes - he adds a lot of 'wrapping-up-plot-holes' speeches, which always serve to highlight a writer's failings throughout the rest of the novel.
The second part of the book, the key part, I'd suggest, was bloated, boring and repetitive - I appreciate that the protagonist has amnesia, but I don't. A better author/editor could have made that section of the book a lot shorter and kept it going along at a decent pace.
I didn't like any of the characters at all; none of them felt like real people. Soap opera cliches, except soap opera characters have a bit of depth to them. The main character, Christine, doesn't have many likeable traits so you aren't left rooting for her or even caring what happens to her. All the characters also spend a lot of time letting out 'animal howls from deep within' and other such delights, which you don't often see around Crouch End, I'm told. I wouldn't like to spend a lot of time in their company, let's put it that way.
The ending was laughable. After however many pages of boredom, to cram the action into that tiny bit of book was ridiculous. The outcome (which I won't spoil) was just complete tosh.
I feel that a stronger writer could have made something slow burning yet deeply unsettling and creepy with this idea. I'd imagine that's why the film rights were snapped up as a screenwriter thinks he could do a better job. I hope he does. However, in this guise we're left with a slow, tedious novel with paper thin characters a rushed and ridiculous end.