As much as my wife and I enjoy formal nights, I am afraid that they are going the way of the Dodo bird.
That being said, there are still many folks who get all dressed up and I have never felt out of place with a jacket and tie on the cruise elegant nights. The interesting thing is that I am the most informal guy I know and you will usually find me wearing a pair of jogging shorts and a T-shirt that I got for free when I am not at work. However, there is something about being on a cruise that makes me want to dress up a bit, plus my wife really enjoys it too!
Anyway, take care and have a great trip!