Maybe the fact that being buried alive is such a common fear kept people from watching this movie, but it shouldn't have. It turned out to be surprisingly interesting with more suspense and action than you expect from a movie that takes place in such a tiny space.
However you feel about Ryan Reynolds, this is a very different type of role for him. Actually putting him in it (though he was quite good) might have been a mistake because of the baggage people carry about him (on the love and hate side). This movie should have received much more appreciation than it did.
A very simple idea, with virtually no need for effects or screen tricks, almost the entire film takes place inside of a coffin. The space is small and there's really only one actor that we see for any period of time. It probably shouldn't work, but it does.
The entire film ends up on his shoulders and he carries it well. A fantastic and simple idea that plays through in an interesting way. Well worth a look.