Embarkation at Southampton was the quickest and least fuss that we have encountered on any of our cruises (all from Southampton), i think this was helped by the fact that passengers seemed to have been allowed on earlier than with P&O, thus spreading out the arrivals. We particularly enjoyed the complimentary champagne on boarding, a lovely touch.(Disembarkation was even faster - no waiting at all!)
We always go for an inside cabin and this was the best and largest we have ever had, it really did seem roomy - helped by the rounded bed. Lots of wardrobe space, slightly lacking in drawer space, but large over bed cupboards enabled all our clothes to be packed away. Also a spacious bathroom with a proper shower cubicle rather than curtains.
The ship itself was lovely and still looks nice and new. Lots of different seating areas around the ship, both inside and out. Slightly cramped on the main sunbed decks, but there were many other areas to sit if you didn't want to be that friendly :)The Staff all over the ship could not be faulted, all greeted us as we passed by and would do anything to help/serve us. They really did seem a friendly bunch.
The only slight downsides that we encountered are all, i think, due to the differences between UK and US/European customers. So perhaps just a question of taste/what we are used to. For example, we didn't feel that there was much to do in the evenings (other than sit in bars or use the casino) - particularly if the "late sitting" show was pre-dinner. Prices were steep compared to P&O - cheapest bottle of wine at dinner - $28/30, cruise DVD $35 - beauty services also dearer. We were also charged for shuttle services, around $10-$15 (free on other lines).
However, perhaps this is the same with all non P&O?
Other than these small gripes we really had a fantastic time and loved the little touches that obviously added the 5th Star. Iced flannels and water when returning from tours, toilet attendants in the general areas and very stylish restrooms, and very attentive staff.
We are glad we tried Celebrity and will be back at some point - maybe after a few more P&O!