I wish I had been on more, but here goes. Princess is my favorite, so far. Since they were acquired by Carnival PCL, they have changed and not for the better. Still, they do offer what I consider, the best cruise experience. It's not stuffy, but it is classy. The crew are professional, again not stuffy, but not as layed back as some of the other lines. The ships are immaculate and the food used to be fantastic. The food now is good, sometimes great, but it has slid since the take-over. I prefer the Sun class ships (Sun, Sea, and Dawn), but they have moved the grand class into Ft. Lauderdale now. We went on the Royal (one of their smallest ships-don't know what the class is called) in December/January and really enjoyed her. She's the size of the old cruise ships where the Sun class ships seem to be just perfect. They're 1950 passengers, so they're not tiny, but they're not huge like the Ruby and Emerald. Not that the Ruby and Emerald aren't beautiful ships, they're just hard to get around (have to go up or down to get to where you want to go).
Costa is a great line, if you don't let some things bother you like: all announcements are made in 5 languages, muster is also made in 5 languages while you are standing outside in your life jacket, not all the crew speak English but you are always able to find someone who does, the dining experience is like a huge Italian family dinner (which I love), and the crew is not quite as responsive as other lines that sail out of the US and cater to Americans. It's not that they don't attend to your needs, it's just that it takes a while. Costa is a more lively cruise, VERY family oriented with gobs of children running around. The ships are beautiful, clean, and well maintained. The food is good, again not great, just good. The cabins are HUGE for a mass market line, much larger than a Princess cabin. (We've been in inside, outside, and balconies on all lines except HAL. HAL was just an inside. Never had a suite of any sort).
Carnival is a fun line. We've sailed on her when she was the "party" line and more recently after they changed their image. They have definitely cleaned up their act, no more really inebriated folks on board. The ships are clean, and the staff is fun loving. They still do the animal towels and candy. They have geared their ships more towards the family experience, have a great children's program, are installing the water works on many of their ships, and have taken the Movies Under the Stars from Princess and put up large screens on many of their ships. The food is good. The chocolate melting cake is divine.
Royal has changed from being upscale to more family oriented. Not sure between Royal and Carnival which has the smaller cabins. To me, it feels like Royal does. It might just be the lay out of the cabin, but it feels much smaller. The food used to be great, now it is good to average. Their ships are beautiful, kept immaculately, service is fantastic. They're usually higher priced than the other mainstream cruise lines for the same itineraries. Guess they have to pay for their rock walls, ice rinks, floriders, and other ammenities not offered on other lines. I haven't had the privilege of sailing on any of their HUGE ships, so cannot compare them fairly with the likes of the Emerald.
Holland America is more upscale. The ships are immaculate with beautiful decor. Service is impeccable. Cabins are on par with Princess as far as the size. They cater to an older clientelle, although they do have facilities for children, it's not their forte. Their food is elegant, but I cannot say in honesty that I was impressed with the flavor. Presentation was beautiful, and some dinners were very good. I guess they don't season highly because they do cater to an older demographic.
NCL is another fun line. Their ships are distinctly decorated. I love the fun colors and whimsical themes. (Costa and Carnival also have great colors and themes-some can be a little overboard-but they're always interesting). The crew is much more layed back. They, too, have a good children's program, but are not geared as much towards that as Carnival and Costa. I enjoy their food, they have more spicy offerings than most lines. But, it's definitely not 5 star food, it is good. Their sticky buns are sinful (on the main buffet for breakfast). The ships are clean, cabins are small, prices are definitely good.
All of the lines I've been on have been unique, and great in their own ways. I've been repeat customers on all of them, with Princess leading the way at 8, Carnival 5, NCL 4, Costa 5, Royal 3, HAL 2. I believe that you can have a fantastic time on any ship when you go with no preconceived notions and go with the flow. I'd love to experience every line, including river cruises. I could definitely find the time, if I could only find the money