At the beginning I found this film a bit boring, but as it starts to develop, it really kept my attention.
Amazing actors and story. Really worth seeing.

And I agree with Paulorsadv: it's a underrated film.

If you like this check out Deathtrap (Lumet, 1982).

This really is a wonderful movie, but I just felt like it was too predictable. Maybe it's because I'm very skeptical, but I just don't think I'm as gullible as the protagonists.