I have only cruised Royal Caribbean and Celebrity and my feeling is when I'm in the mood for a little more glitz then I book a RCCL mega ship.
When I'm in the mood to relax and just want to chill with good food and drink, I choose Celebrity. As a matter of fact I just booked a 10-night Summit So. Caribbean cruise for $85 a day. I think I got a very good deal on this cruise. I feel their food is step above RCCL.
I'm glad Celebrity and RCCL are a bit different in their class of service, because I can afford these lines comfortably and I have a choice.
It's all a matter of taste and where you feel most comfortable being. I know I'm not Crystal material, so I wouldn't book a cruise on a line like that because I just wouldn't fit in.
All I can say is I'm glad we live in a country where we have so many choices and we can enjoy cruising to our heart's content.