Fun fact: on several occasions, I have had convos with people extolling the virtues of cruising wherein they responded, "Meh, I tried a cruise once and didn't like it." When I asked which cruise line, every single one of them said Carnival and it was usually one of those 3 or 4 nighters. So, first of all, the 3 & 4 nighters are TOTALLY different animals from the 6-8 (and more) nighters. Different experience, different quality, different, um, "clientele."
Secondly, Carnival has a reputation for being "low quality" - altho I have a cousin who cruises as much as I do - if not more - and they swear by Carnival, but of course do longer trips, are high point frequent cruisers and get perks, etc. I had never sailed it myself, but when I won a free cruise a couple of months ago, I thought, "Finally! I can see if their ire is well-deserved!" Color me surprised!
First of all, the entertainment was top notch. Possibly the best of any major line I've sailed. I started cruising with sub-contracted gay cruises back in the 80s, so was spoiled by entertainment like Cyndi Lauper, Melissa Etheridge, Rosie O'Donnell, Sandra Bernhard, etc. Since that time, with a few exceptions on NCL, I've found cruise ship shows/entertainment......lacking. But I went to 4 shows on Carnival and loved every one. Was there the occasional "off key" note from a singer? Yep, but not awful or frequent (after 3 covers of Journey, I DO now have a better appreciation for Steve Perry's voice). Their holiday show, in particular, was amazing and heartwarming and inspiring all at once. So big points there.
The service was standard cruise ship excellent. It is SO nice to have folks cleaning up after you! Clearing your plates, making your bed, cleaning your room, etc. And they were good w/o being overly obtrusive. The ship - The Dream - was well-maintained and had lots of stuff to do. Best mini golf I've seen on a ship. Multiple water slides, a good library, lots of bars/clubs/cubbys to hide and read, etc. Lots of trivia.
The food was not the best, but not the worst - and while their buffet was definitely lacking, lunch at least made up for it with a lot of "quick service/made to order fresh" food options: I don't think much of Guy Fieri one way or the other, but they had his Burger bar available every day from Noon - 6, and his "Pig & Smoke" BBQ spot, as well. They also had a VERY good burrito bar, a taco bar, a Deli with over a dozen hot & cold sandwich options, a Mongolian BBQ, fresh made pizza available from 11am - midnight, and a great made-to-order Pasta bar. The breakfast buffet was unremarkable, but breakfast in the dining room was good. Dinner was sometimes good and sometimes average. Service was great.
So! How does Carnival compare to the other lines I've sailed? See below! Disclaimer: perceptions of a cruise line can totally be colored by the ship you sail with them - how new, how well-maintained, amenities, etc. So keeping in mind that NCL is the only line I've sailed with more than once (WAY more), take my rankings of the rest with a grain of salt - altho I will say they tend to generally match the "standard industry gossip"