Have not been on Diamond Princess yet but did the 7 day Mexican Riviera last March on Sapphire Princess, her identical twin. I haven't yet been on RCI or Carnival so I can't make a comparison; but we had an absolutely terrific time on Princess. We tried a mini-suite for the first time and we're spoiled forever; that sunrise coffee and sunset cocktail on your own balcony is just priceless; and the larger bathroom, closet and sitting area are more than worth the slightly higher fare.
The ship was immaculate, the crew genuinely cheerful and attentive, the dining rooms and entertainment excellent, and the shore excursions well organized and punctual. I could go on and on, and usually do, but I'll just say I've cruised princess for over 30 years and haven't been dissapointed yet; and Sapphire Princess was definitely one of the highlights.
But however you go, have a great cruise.