While I do not understand why Emily Mortimer was used in this (I find her annoying in general), the rest of the casting was done really well. Ryan Reynolds does a great job with his role of a man who has his whole life shaken up after a bizarre (and comical) set of circumstances reveals a serious and life-changing fact to him.
How he deals with this revelation, and his family and closest friend deals with it, is both entertaining, and a bit moving. When I first saw this movie, I recommended it to a lot of people while I worked in a video store. None came back with any complaints over the choice, but I think it's now become a lesser known title overall, which is a shame, because it's a pretty solid movie.