We have cruised Carnival, Norwegian, and RCL and our last two have been with Princess. We had a great time on Ruby Princess last year, but everything on the new Regal was better. Granted we had a balcony this time instead of a porthole (I'll never cruise again w/o a balcony). But right down the line there was improvement - the entertainment/shows, the food, the service, and of course...the ship.
I never held the ship-board worn spots against any cruise line...we can make our fun anywhere...but there is something to be said for New Everywhere. As awesome as RCL's Liberty was, we had stains galore in our cabin and there were other signs of age throughout. Not so with the Regal, and that'll probably affect my judgement of future ships going forward.
The crowd is mostly retirees, but my wife and I are in our 50s so it almost makes us part of the younger set by default (heh). I think if I went back to a Carnival ship with a lot of daytime partying around the pool deck I'd be more annoyed. Everything about the sea days were relaxing, although a little more music instead of instead of 10 year old movies would be preferred. The Piazza is elegant and a great place to while away the evening hours with a cocktail. The shows were first rate, and the CD was a pro of the first order.