We have sailed on the Mercury, Century and Millennium. All three ships had their advantages. We preferred the decor on the Century; the design of the Mercury was better for walking around on outside decks; and we thoroughly enjoyed the Millennium with its exterior elevators and Olympic Dining Room. We are presently planning another cruise on the Millennium, but wouldn't hesitate to sail on any of the others.
What we heard from staff is that it is harder to work in the Millennium dining room than in the Century class ships. They really have to hustle to serve meals and even then it takes about 30 minutes longer to accomplish the same thing on the Millennium as it does on other ships. If you notice early seating on the Millennium is at 6:00 instead of 6:15. You get the feeling that they have less time to just chat with you unless you go to the dining room for lunch for this reason.
At any rate, on all three ships we received excellent service, very good food, and enjoyed all the little details that you expect on Celebrity Cruises.