Very nice indeed. The mystery is great and the writing is flawless.
I guess years of reading detective manga like Conan or Kindaichi kinda help. I'm sure that they were inspired by Agatha's books. This book has some elements similar to one of Kindaichi's cases and I got some few points right. But well, I'm sure too that without having read Kindaichi beforehand, it will be impossible for me to figure it out.
And then more than just about mystery and thriller, Christie talks of something bigger - justice, as well as criminals' psychology. Such a small book and yet it does many things easily, effortlessly.