I have been on Princess, NCL, and Carnival. Celebrity is BY FAR superior to all in my opinion, especially the new millennium class ships. I can't imagine how someone cannot be impressed with them. The ships are beautiful and elegant, the rooms are spacious (and I always get a standard outside), the staff is good to great, the food is very good, and the entertainment is getting better all the time. It is not a 'party' cruise line so if that is your thing I can see you might be disappointed. But if you want elegance for a reasonable (although lately rising) price, I feel you can't beat Celebrity.
We have been on Celebrity 12 times and have two more upcoming. The Constellation in particular is beyond my wildest dreams!
To each their own, but I think Celebrity is 'simply the best'.