We are a 50-something married couple with 12 cruises under our belts, so, I think we have enough experience to be objective. We have sailed on Royal Caribbean, Carnival, Princess, Holland America, Celebrity (Mercury), and Oceania. After a day or two onboard Millennium, my husband looked at me and said, "I think we've finally found what we've been looking for". After all seven days and nights, I agree completely!
Embarkation was very quick, we got there at 11:00 am, and boarded this ship 15 minutes later. Celebrity has a better system for checking people in than the others. When we got onboard, we were greeted with a smile and a glass of champagne, and escorted to our cabin. We had taken advantage of a deeply-discounted special and booked the lowest category cabin 2180 inside, hoping to upgrade at the pier, but the ship was sold out so we couldn't do that. As it turned out, the cabin was adequate for us, having plenty of drawer and closet space (and, believe me, I'm a heavy packer!) and a bigger shower than most. Our cabin steward was Miguel, he was excellently efficient and so pleasant to deal with.
The ship is gorgeous, elegantly decorated, and well designed layout. Our tablemates were interesting and friendly (they were two couples who have had 44 cruises on Celebrity each--and we gleaned their knowledge over the course of the week about how to get the best price, cabins, etc-- and, by the way, they had all traveled on the other lines, too, and they were all convinced that Celebrity has a superior product--they were retired people in their 60s).