I have just returned from a P&O Christmas Cruise of the Caribbean, and also read a review by KevinML who appears to have cruised the same. I would concur with much of his comments but disagree with others.
Briefly, the dining room staff, as always, were excellent; the only problems I witnessed were rude guests who treat these hard working people very poorly. Dinner, I agree, now appears to be of a lower standard. Plate service rather than silver service. Portions are smaller, no doubt to save cost and waste, and they will also serve you more/extra if you so request. No one goes hungry on a cruise!
Formal nights: for me 4 is not enough, I would do formal nearly every night. The number of formal/casual nights are disclosed well in advance and can be found on your own cruise personaliser
NOISEY BEYOND BELIEF, 24/7. I WOULD RECOMMEND THESE ARE AVOIDED, MID SHIPS.