Having cruised with a number of cruise lines - we returned to P&O this year.Let me say now that if you want to find fault you will find it - remember this is my opinion of my holiday. I post a review after each cruise and I am honest / fair. Its a shame that there have been many poor reviews.
Staterooms - P&O do it well overall and for good value. We had two staterooms which believe it it or not was a cheaper option than us all in together. We booked early - had a good on board credit and free parking.
Booking early - Do think about this when booking and as a select fare payer we had free transport when in port. There's no hard sell and prices are pretty much what you'd pay in UK. We always pre=order our wine package and we saved around £40.