Royal Princess is not a boring ship - they have some kind of entertainment happening in the atrium all the time, and then free OnDemand movies in your stateroom.
The staterooms are very comfortable, the food is good.
I think the thing that makes them unique is that they are really mainstream, pretty big ships, but they have great itineraries and tours. Their Med cruises are great, for example.
But the ships are not as exciting as Royal Caribbean, and not quite as posh as Celebrity, and more crowded than Holland America. Royal Princess is a beautiful ship.
And in fact they have a VERY loyal following, I think largely because they have a very consistent product, rarely do I see anyone say 'we went on Princess and never again!"