This was our first cruise with Celebrity (we are regular Royal Caribbean cruisers) and my overall impression is that the step up in cost is not matched by a step up in quality. Our boarding experience was good as we had arrived quite early, but from speaking to fellow cruisers, we were very lucky. Heavy overnight rain the night before caused a serious power outage and Celebrity's response to their customers being left for hours packed on ill-equipped coaches or stuffy tents was very poor.
What disappointed me most was the service on board. Bearing in mind that every time a purchase is made on board a whopping 18% service charge is added to the bill, you might expect this charge to be matched with excellent service. This was certainly not my experience. In particular, the bar staff were not only unfriendly, but often, rude and unhelpful. On one occasion, I ordered two glasses of red wine and the waiter took my sea pass card and went off to get our drinks. It took him over 30 minutes to return with the wine. In between times, I went off to ask what the problem was and was told "there is a problem with your sea pass card". This was untrue, there was no problem and I still don't really know why it took so long.
On the plus side, they did switch ports at very short notice, when bad weather in Villefranche would have meant a wasted day there.