My husband and I have cruised on all 3 ship classes. Have been on Century 3 times (our favorite), Horizon twice, Zenith once & Millennium once. We felt the most comfortable on the Century, and even though the Horizon & Zenith are smaller ships without an atrium or balcony cabins, we enjoyed those ships also.
The Millennium is really a beautiful ship, and it was easy to find your way around even being so large, but she was lacking the warm feeling of the Century. Its hard to put your finger on the reason, but even saying that, we have booked the Summit for an upcoming 10-night cruise because we like that itinerary. Also, we love Celebrity's excellent food, service & entertainment.