Not sure what you mean by "premium", Paul, but we sailed Millennium in November & thought she was in great shape, & an elegantly appointed ship. Our previous cruise was on Century, & dispite her being 14+, I thought that she was just as elegant. We haven't sailed the S class, but I'm not really hung up on newer is better. We've found that the cruise experience on all Celebrity ships is consistent throughout the fleet.
When I look for a sailing, if it's on Celebrity, I look for the best value that fits our schedule & where we are interested in going at that time, regardless of the ship. We have enjoyed all of Celebrity's ships. I'm sure we will try the S class someday, but probably not go out of our way to do so.