I sailed her 2 years ago. I also sailed the Century and Galaxy, toured the Mercury, spent afternoons onboard the Zenith and Horizon and toured the Millennium. Celebrity's decor is one of my favorites of the mass market cruise lines.
There's nothing unanimous in life, and you did ask for favorite and not so favorite parts. So, I'll give you my honest assessment of the Constellation. Sorry that it is so long, it is like a mini-review.
I'm sorry, I like her least of all of the Celebrity ships I've been aboard. She is very pretty. No doubt. She has quality furnishings. But those big, uncrowded areas, while nice, seemed oversized for me. I like large areas, but balanced, with smaller, more intimate ones. Humongous ships like RCI's Voyager class have smaller public areas than the Millennium class, like the pubs they have in their Royal Promenades and their champagne lounge areas. And even larger areas, like their schooner's lounge, are divided nicely into different areas. I've been aboard the Carnival Legend, a similarly sized ship, and while I found her decor beyond ugly and uncomfortable and repetitive, and her ceilings low, I thought she had more variety in both size and layout in her public rooms. Also, the problem with the Millennium ships is that they are all so similar, most people are bound to like some area of the sistership a bit more than the corresponding area of another. I think, compared to Millennium, Connie's not as elegant. Where Millennium has crystal light fixtures Constellation has etched glass, where Millennium has raised carpet details Connie's are flat and patterned, where Millennium has elegant lamps on her side tables near her martini bar, there's none on the side tables of the Connie. That's not to say Connie isn't elegant, she's quite elegant, especially Ocean LIners restaurant, her casino, and, like the other Millennium ships, her Michael's Club. I think things get too busy in her Rendezvous, too many patterns compared to the more soothing treatments on some of her sisterships. I also find her foyer's carpeting jarring.
Staff is variable, we had problems on our Connie cruise but that's not due to the ship, it was luck of the draw. A friend of mine that was on our Connie cruise cruised her again and said everything was impeccable. Service, food, all first rate, way past what we experienced. That's the right way to go
My favorite Celebrity ship is Galaxy. Varied public areas, not crowded yet ship brings pax together, vibrant colors, unique decor not shared by other ships, 13,000 tons smaller than the Constellation but uses her spaces more efficiently. The dining room with large picture windows were first introduced on Century and indeed Galaxy has these. Also, almost all of what was mentioned in service and style is part of all Celebrity ships, not just the newest ones. Only specialty restaurants and Cirque de Soleil on two ships and spa food are unique to the M ships.
I'm just not a fan of the Milllennium ships, to me there are numerous design flaws. Only 2 dance floors on the 91,000 ton ship where there are 3 on the 48,000 ton Zenith, long wings in the dining room were diners in these areas feel apart from the rest of the dining room, H-shaped lounges where people in the "corridor" areas feel apart from those in the middle area (Rendezvous and martini/champagne bar directly above it), lack of different levels in lounges and conversation nooks, observation lounge too large and should be subdivided (and has been to accomodate Cirque de Soleil), way too many shops and space devoted to them, underutilized areas that house meeting rooms (that artiicially inflate its passenger space ratio), plant shops that are sports bar on the Millie and should have stayed sports bar (IMO)--these may be going as they aren't producing the expected revenue, library has no writing tables (mind-boggling oversight). Already, several things have been changed about these ships which I think reveals their design flaws. Michael's Club, originally a cigar smoker's lounge was changed into a piano bar. I feel for 2 reasons. One, she was oversized for the cigar lounge purpose. 2. The ship needed another entertainment venue and one that was moderately sized and not as big as the other lounges. The t-pool turned out to be a fiasco at one time. It was being overrun by unruly children where it was supposed to be a peaceful, relaxing oasis. By making it larger, the jets weren't strong enough to provide the same effect as in the smaller versions on the Century ships. To address the crowding in the area, Celebrity charged for a short time for the area. The feedback wasn't friendly, to put in midly. So now, they are left having to police the area. It isn't a bad area, but I think it pales to what's on the Century class ships, a private, peaceful oasis that you do pay for, but I think it is worth it.
Overall, I trace the problem to this fact, and this may be much more apparent to those that have sailed all classes of Celebrity ships and have been sailing Celebrity for awhile (and to some degree not charmed by the newness and apparent luxury decor of this class of ship). While the Horizon and Century ships were designed for Celebrity by Celebrity for a Celebrity lifestyle, the Millennium ships were designed using Celebrity designers and adapted to a shared RCI hull. Therefore, certain position of rooms had to change, the dining room's galley location was changed, and the design fits a not-as-well-defined Celebrity lifestyle on an adapted-for-Celebrity platform. But it is also true, these ships are fairly popular, and many folks just don't have the problems with them that I have with them.
Something that has been stated that isn't necessarily true is that Millennium ships have the highest PSR in the mass market lines. HAL beats the Millennium ships with their Vista ships. They are 1850 pax, basis 2, and are about 85,000 tons.