Yes, it's a classic for a reason. Moreover, it's so well-known, most people are aware of the ending, even if they haven't read it.

Watching you puzzle it out as we went along, completely unaware, and come so close to reaching the right conclusion... I admit, I felt a bit of a vicarious excitement for you as you did so.

Perhaps we'll do this again? We can move on to The Murder of Roger Ackroyd... no, on second thought, Murder on the Orient Express is a much better work to introduce you to monsieur Poirot.