I liked the concept of this very much, but I don't think Ryan Reynolds was up to the job. On the one hand, he excelled at being the 'everyman' that this kind of film calls for, but he's not an acting heavyweight, so it didn't grip me as it should. Someone with more acting chops/gravitas would have made a better choice, imho.
Some of the cinematography kinda irritated me - it made the coffin look all kinds of sizes, which really diminished the film - it depleted the sense of claustrophobia, which was integral.
Kudos on the ending, which was just great. All in all, a really neat idea, which needed a slightly bigger budget and more experienc3ed director.