I absolutely LOVE cruising. I book one or two or three every year. It's my "go to" vacation.
Now to answer your question. The mass market cruise lines, RCCL, Carnival, Princess, Celebrity, MSC, NCL, etc. or more alike than different. Think of it like department stores, JcPenney, Kohl's, Sears, Belks, Macy's, Dillards, Sak 5th Ave, Neimans, etc. Each dept store sells clothing, bedding, small appliances, luggage, etc. Depending on what you're looking for and when you're looking for it you may lean towards one store over the others. It's the exact same thing with cruising.
Every cruise line has that lil' something special that depending on when and what you want in a cruise one of the 10+ cruise lines will have it. I love options so cruising is always an option when I'm looking to vacation.
My fav cruise line is RCCL. My fav ship is RCCL Allure of the seas with the Anthem OTS a close 2nd. I like RCCL because it was my 1st. 20+ cruises later it's still my fav but I've been on RCCL, Princess, Carnival and Celebrity. Same product but each line has that thing that I love about them so when I'm shopping around I look for what I want to get out of the cruise - itinerary, ship, port, sea days and select accordingly.
RCCL new mega ships are polished and refined works of art to me. Love the broadway shows, park cafe and hot dog stand, amphitheater should not be missed, the different neighborhoods keep the crowds light on a regular sea day, CoCo Cay hands down the best private island I have visited since the revitalization. I enjoy all the innovation and amenities RCCL offers. I always look at RCCL first when shopping for a cruise.
Carnival - Hubby loves Guys burgers and the Blue Iguana, the taco shop. We both usually like the comedians. Price - we can usually find Carnival cheaper out of our Galveston or NOLA home ports. We can purchase after final payment since we don't have to purchase airline tickets; we're single no kids so can cruise out of season and drive to the ports.
Princess - they do Alaska wonderfully. And when they were in TX they were at our home port of Galveston.
Celebrity reminded me of my regal and sophisticated aunties home. It was nice and lovely but I needed a tad bit more "fun" options. Do you have those relatives who come from money and you pulled names at Thanksgiving and everyone has a name to purchase a gift. You can always tell they spent more than the $25 limit because the gifts at Nieman's and Sak's start at $200. LOL!!! Celebrity is one of the cruise lines those relatives would cruise. Absolutely nothing wrong with Celebrity but it is a bit more refined.
FOOD - They are all your typical cafeteria food in the buffet lines and your banquet food in the main dining rooms. I don't care how you look at it, it is what it is. Is the food good across all the lines? YES! Will you go hungry? NO!! It blows my mind when people say the food was gross all week long. I just don't get it. Most ships have 3-6 venues open most of the time. So out of 6 restaurants you couldn't find anything to eat and everything was gross. SMH!!?!? I really want to know what these folks eat at home. Last week I made spaghetti and ate it for 3 days. LOL! I would have loved to have 6 options on Wednesday, when I was sick of spaghetti.
These are mass market ships not fine dining Michelin 5 star restaurants. If you're looking for fine dining book a Paul Gaugin cruise. Mass market = tacos, burgers, pizza and pasta, salad and chicken which you will find on all these cruise ships plus some.
O and if the food is so gross why does most people gain a few pounds after every cruise?!?!? OK getting off my cruise food soapbox.
Like other's have mentioned definitely check out cruise critic. You will learn more than you ever wanted to know about cruising. And remember the negative and positive reviews are a very small portion from a very small sample size of cruisers. Cruising is like Disney, you will fall in love with it and become addicted or you will hate it and a few who don't care either way.
Happy CRUISE shopping!!!