There are several very attractive points to the book. First, the case takes Poirot and Hastings out of their usual socio-economic comfort zone: three of the four murders are in lower middle class or working class settings, and Christie largely reverses her usual view of the universe where poor people are normally invisible. Second, the fact that the villain sets the story up as a battle of wits with Poirot from the start of the book gives it a completely different dynamic: it's not a case of Poirot inserting himself into someone else's tragedy, instead he is dragged into a nefarious plot from the very beginning, and it is a little gratifying to see him lose the initiative (though of course we cheer when he regains it). And finally, the speculation on the mind-set of the serial killer, in a novel written and set in the mid 1930s, reminds us that this is a topic of horrified fascination that has been around for a long time. Oh yes, and the plot is well constructed and the solution reasonably fair.
This was one of the ones I had read as a teenager and had fond memories of; and I was not disappointed to return to it.