As a lover of the mystery genre, I'm happy to have read this early example of a closed-room mystery story. I can see its influence on Agatha Christie and on the genre itself. That said, since Hercule Poirot himself calls this story "very nearly" unfair, "but not quite" (see The Clocks), I think I will go ahead and just call it unfair.

Can the reader suspect the correct person, given the clues? Yes, at least for a few minutes. But can the reader have any inkling who this person actually is, what his motives might be, how many layers of history the story has? No, not until our resident Boy Wonder spouts the whole thing in the final act. I realize the Expositional Big Reveal is often part of early mystery stories, and maybe I'd mind less if I didn't so strongly dislike amateur detective Rouletabille (for being such a ridiculously special genius) and narrator buddy Sainclair (for being so pointless and useless I forgot his name).

Conclusion: it's a solid story told less-than-fairly, a really smart mystery populated with flat characters.