I actually really like this one, I know a lot of people don't think it is one of AC's best.  But I enjoyed that this was a bit different from her other works.  The serial case adds a sense of pressure and urgency to Poirot, he is not all-knowing and with each murder, he is failing.  It was less 'Sherlock Holmes' seeing clues we can't and more traditional police work, running down leads and interviewing people over and over, finding the commonalities.  I also loved the psychoanalysis of serial killers, great research from AC from the nature of serial killers (and surprising how much is still true from 1937 to today).  AC took inspiration from true cases in the past (Jack the Ripper taunting of police) This is not much a who did it, but a why did he do it, and how is he going to get caught?  I enjoy the unusual combining of the first-person narrative and third-person narrative.