A couple of opening caveats: This was only my 2nd cruise, and both of them have been on Carnival. So my cruise experience is limited and I cannot compare it with other cruise lines.
About the ship: To say it's immense is an understatement. It is freaking enormous. It's very easy to lose track of where you are inside of it, at least during your first couple days aboard. The layout of the ship leaves a little to be desired, too, since it has some odd quirks. For example, Deck 3 has a huge blocked-off area immediately in front of the dining room; to get to any other part of the ship from there you have to go up or down one or more decks first. Second, there's nowhere near enough elevator capacity; at times (especially during disembarkation) the elevators basically cease to exist, they're always crammed and you can't get in. This wasn't much of a problem for me, but I imagine elderly folks trying to go up several decks had a heckuva time doing so.
Next, the food was fantastic. We mostly ate in the dining room (for reasons I will get to momentarily). The food was fantastic and the service there great. We never had a single problem with anything we ate; it was almost uniformly terrific. As for why we ate in the dining room all the time ... the Lido Deck buffets were exceptionally busy, often with lines, and they frequently ran out of things. Even when there weren't lines, though, there were an awful lot of people running around and it was so congested that you couldn't get around. After only a short time trying to deal with the madness that was the Magic Lido Deck, we all basically gave up.
The service outside the dining room was as good or better. Our cabin steward was helpful and attentive. She knew us by name and remembered all our preferences. Everywhere we went, staff said "Hello!" or "How was your day?" or "Have a nice day!" etc. Smiles all around and as much help as you could ask for.
Activities were decent, but I expected more on a ship as big as the Magic. Several shows in the big theater were good, including a 70s/80s revue which was surprisingly good. A special Christmas show was also decent. But not all of them were that good. A "murder mystery" show left a lot to be desired. I saw 4 comedians in the lounge, they were all competent, but only one was truly hilarious.
We had to tender into Belize City, which didn't work out all that well. They were late getting started and they clearly had far more people leaving the ship than they could deal with (although I'm sure Carnival was aware there c. 4,500 passengers aboard). They were late getting started, and anyone who wasn't on a Carnival-sold excursion had to wait a LONG time to get to port. Tender boats came and went at 30-minute intervals at best, and that was nowhere near enough. Carnival either will have to beef up its tendering operation, or simply stop sending the Magic to tender-only ports. It's that simple.
Getting back to Galveston was also hideous. They were very late starting with disembarkation and it took forever. That's not entirely Carnival's fault ... there were only 4 (yes, 4!) customs agents handling those 4,500+ passengers. Again, Carnival has to reconsider bringing a ship the size of the Magic to a port that clearly doesn't have a large enough customs staff to handle it.
One last thing: Passenger etiquette left a lot to be desired. During this cruise I saw people literally climbing over others trying to get into elevators (while folks were trying to get off!). I saw passengers barking orders at, and otherwise being rude to, Carnival staff. The last night of the cruise, there were some kids running up and down my cabin hallway, yelling, banging on doors, and playing some sort of game. I saw people clogging up halls and walkways all over the place. It was not pretty ... but it also had nothing to do with Carnival. I get that this ship was huge and congested, but really, that's no reason to lash out at other people or act like boors.