I'm really looking forward to this film being released; I think it should come out in the UK by May (not sure about the US). I was really annoyed when they put off the release last year. I myself see absolutely no good reason for it, especially when you consider how the popularity of cinema increased at the end of last year.

As for the casting, if you ignore a couple of films, Leonardo di Caprio is a good actor, and I believe Scorsese is the kind if director who can bring out the best in him. Cameron Diaz is also a good actress (look at Being John Malkovich). Again, Scorsese can bring out the best in her. Daniel Day Lewis will be excellent regardless.

Also, remember that this film is dealing with a period and aspect of American history which hasn't been explored in great depth before (of course, The Godfather Part II touched on the history of immigration).