While our favourite line is Princess, we very much like RCI too. We did one disastrous 35 day cruise from Fremantle to Rome with MSC and in all honesty if I won another cruise on that line I would find someone I disliked intensely and give it to them.
I think it's about finding a cruise line that as far as possible meets expectations. I taken seriously advice from friends who have warned us off P & O and also Cunard. We're casual people, we never do formal nights and eat in the MDR once in a blue moon. We like to spend our sea days reading, engaging in any and every trivia session possible, attending destination lectures and generally relaxing either on our balcony or a less patronised deck. In the evenings we prefer to watch a movie and occasionally if the show looks particularly good we'll go the early performance. So, we're not party people or late night people and Princess / RCI suits us perfectly.