I didn't realise I had already read this one until about a quarter of the way in, which is pretty good for a mystery. Particularly one as good as this one. Here's why:

I thought I remembered who the villain was but as I was reading it, I tried looking for clues or details that may have given it away or that contradicted the conclusion. I couldn't find any. This had me doubting my memory until the very end. 

Christie's brilliance also shows itself in the characters. There isn't much diversity, yet each person is distinct. Inspector Battle is completely different to Poirot or Marple, yet likeable and reliable. 

The way she pulls you in different directions with her plot twists is exciting rather than dizzying.

I've increased my rating from 4 to 5 because I have enjoyed rereading it so much and I would read it again in another several years just to see if I get tricked again!