Normally, I don't need to like a character to find it engaging. Think Olive Kitteridge, a less than charming character, but in the long run, at least interesting, with some redeeming qualities. In fact, I've often been surprised when someone has told me he/she disliked a book because he/she didn't like the main character. And then I encountered MI5 agent, Serena Frome (rhymes with Plume). Vacuous, insipid, self-centered, opportunistic - a bore. I couldn't imagine spending several hours with her. Add the fact that the outcome of this tale is told in the very first paragraph, and I'm surprised I lasted as long as I did.
Now, having said this, my brother, an avid reader and a McEwan fan, did finish Sweet Tooth. His assessment - enjoyable, though not as good as previous novels. So, if you decide to give the novel a shot, I'd be interested in whether you enjoyed it?