I tried other lines when I got a great price or good Itinerary that my favorite does not go to. Cruise lines are like Cars, what you want when your young and single is not what you want with a family and as you get older you want more luxury and comfort. Pick the cruise line that fits your place in life.
One of the advantages of RCI is that your loyalty program level on RC will get you the same level perks on Celebrity and Azmara. At lower levels this does not mean much but when you hit Diamond you get some free laundry and a cocktail party every night before dinner.
I started out with RC and was brand loyal to them. I tried Princes and did not like it, the food was not good, but this was 20 years ago. I tried Celebrity and loved it. it is now my favorite cruise line. I did NCL and its food and service has it is at the very bottom of my list. I tried HAL because I got a killer price, to old of a group for the age I was when I was on it and now they have lots of smokers since they allow smoking on balconies. I stayed away from Carnival thinking it would be a floating mobile home park until they had a promotion that was so low priced I tried it, it is a blast for a short trip.