Hi. We have cruised, Carnival, Royal Carribean, Princess and my husband has experienced Costa too.
Of these I would rate Princess after Costa and Royal Carribean.
This may be due to our cruising at the hight of the season (Our wedding anniversay being 7th August). I will compare between RC (Royal Carribean) and Princess.
Princess have the extra wide terraced balconies, which means that from Carribe deck down to Emerald the balconies are half exposed to those above. OK if you don't mid the goldfish bowl lifestyle... Also, if you are in a cabin near to the front you will also be under the scrutiny of the bridge crew as well. RC, are smaller balconies, but, they have real wood decks, rather than the plastic tiles (rather hard on the feet), and they are private.
The rooms. Princess have reasonable sized rooms, plenty of mirrors to make them look bigger. However, in the room we had the walk in wardrobe (actually it is an open rail, no doors) is right in front of the bathroom, so when the door to the bathroom is open it blocks the wardrobe and shelving area. RC have sliding doors to the wardrobe and enough space between the bathroom door, to use both areas comfortably. The bathroom decor of RC, in my opinion is nicer to look at, Princess have gone all out with the cream tile and corrian with pale blue accents, a bit old fashioned.
The bed. Princess take the two twin matresses and push them together, RC make the bed with a queen mattress, very much more comfortable.
The TV/bill process. RC enable you to check your on board account using the room tv and remote, Princess make you go to the Pursers desk to get your bill printed out - none of that delivery to the room before you sign to pay up. Also, the tips are automatically added to each days bill - per person, so if you have any question with the service, you will not have the option to set the tip yourself, you will have to contest the tip at the end. RC still allow you to pay the tip separately from the room bill.
Room Service. Princess definately get first place here. Room service was punctual, and accurate, also the food arrived fresh.
Public areas. Princess seem to think that all public areas should have musak playing, so don't expect to hear the sound of the waves or the wind - unless you are confined to your balcony. Also, the promenade deck on Princess is metal, painted to look like wood planks, how tacky. The interior areas are smaller on the Princess ship, they have devoted more space to the shops and trying to part you from your money. RC is not as mercenary, they do not have 'art' auctions every day they can, nor, keep the ship at sea to do this (really, it does not take a whole day and a half to go from Monaco to Barcelona, unless you want to fit yet another auction in). There is an adult only pool and sitting area specified on Princess. But, as it is not an enforced rule, do expect to have to listen to unattended children running amok in this area, as well as every other. Again, this could have been due to the time of year we went - won't make that mistake again.
Food. Princess have the 'personal choice' dinning option. Convenient on a port intensive cruise, but, the two specialist dinning rooms serve the same menu throughout the cruise. The main dinning rooms are fair, to middli'n, nothing spectacular, both lines offer 'seconds' or if you want to eat just desert, go for it... The buffet was not consistent, some items were very good, others were bad. Bit hit and miss.
Public facilities. Yes, the jacuzzies are open 24/7 on the Princess ship, except the day of our anniversary (!) Apparently the ship prepares for the next herd's arrival, 3 days before, and as a result they close all the jacuzzis at once for 'maintenance' (or as one of the crew stated, 'We have so many kids on board...') The casino on any cruise ship is tight, don't expect to win the cost of your cruise here. After all if you don't like it, what are you to do?
We don't normaly do the shows, so I can't comment on the Princess versions.
Safety. We did get to see the Princess theater, as this was the area we were taken to for the safety briefing - they told us where we should muster and let us try on the life jackets - the idea being that unless it is a sinking emergency, this will be the muster area. Of course that's if you survive the trampling that will hapen should there be a for real emergency. I felt safer with the more traditional approach from R C. At least they took a roll call, rather than the princess idea of sending people to check the rooms - somehow I can't see members of the crew volunteering to check rooms as the ship gradually slides below the waves!
Port dock locations. Princess seem to favour the cheaper berths (the one furthest away from the town area) that then enable them to charge you to use a coach to get into the port town itself. They even charged to use the tender service to get into Monaco. RC are able to dock either close enough to town to walk in, or, they provide FREE shuttle buses, and tenders to get into the port. Princess do provide a representative in the port town to answer quiestions as you get off the coach. On the Med cruise we took this year, we did notice that in every location RC had prime locations for their berths.
Tours. Normally we prefer to go independent, but we did opt to take one tour with Princess in Istanbul, which was a reasonable value for money, although the guide had rather a thick accent and the group was rather large, so there was much jostling to get close enough to hear him. Beware, the 'carpet demonstration' is actually a carpet selling 'demonstration', don't expect to see weaving or dying on this, I think the guides uncle owns the shop...
Getting on and off. Both comparable queing experiences to get on, despite the 'express check in' we had requested from Princess (along with everyone else, makes a mockery of the idea really.) The disembarkation was smooth on both cruises.
The Golden Princess is only 2 years old, but is looking a little worn - carpets with bare areas, cracked marble tiles, worn brass trim, and the spa pool needed a good steam cleaning. Fair enough, this was towards the end of the summer cruising, but, the ship should be presented in the same state for every cruise, after all we are paying the same, if not more for that to be the case. The Voyager of the Seas our RC cruise was less obviously worn, although the ship was at that point in its life of a similar age.
Well, hope this is of use to you. Overall the Princess cruise came in second to Royal Carribean, on most points. The time of year can only be blamed so much, there comes a point when it is up to the cruise company to ensure the consistency of the product regardless of the time of year the product is being offered. Of course it could be the fact that Princess has been taken over by Carnival Cruises...you notice I have not talked about the Carnival cruise we had...?
Hope you have a great cruise.