We were signed up to go the the Bahamas and Turks and Caicos, but we were re-routed to Saint John, NB and Halifax, NS due to Hurricane Matthew. I was very pleased with how Carnival handled re-routing us: we were given a couple days' notice and the opportunity to cancel without penalty, or we could come along on the new itinerary and get $100 pp on-board credit and 25% off our next cruise. I thought that was pretty generous! Apparently a good number of people canceled, because the boat was only ~2/3 or 3/4 full, even though it was sold out the week before! The people who came along seemed to be interested in making the most of it, and we found other passengers to be quite friendly!
I can't say enough wonderful things about the staff. Everyone went above and beyond to remember our names and preferences (ahem, drink orders). The staff was all very friendly, and they clearly worked REALLY hard--it was impossible to walk around without seeing someone cleaning something somewhere.
As far as food goes, the buffet on Lido was fine, but nothing to write home about. Burgers at Guy's were delicious. My favorite discovery of the cruise was to get the fries at Guy's with a scoop of chili on them, then walk over to the Blue Iguana to top them with salsa, cheese, etc. Yum! Main dining room food was quite tasty at dinner. For brunch, it would be great if they could rotate the menu a bit or include a daily special. I liked the food better there than the buffet, but it got repetitive. We also occasionally had issues with the food arriving in the MDR cold. We did the Chef's Table, as well, and we LOVED that! The food was great, and the wine was plentiful.
Cabin was fine--3 closets provided plenty of room to hang our clothes. Shower was small, but adequate. Balcony was too small for the chairs that were on it--the chairs took up the entire footprint. Good location for the cabin--close to a stairwell, but not noisy. It was a quick hop up the stairs to get to the Lido deck bars and the spa.
Reasonably walkable city. Citadel Hill is worth a visit, and if you're there at noon, you'll get to hear them fire a cannon, which was cool. Swing by the Halifax Distilling Company for some tasty rum or Garrison Brewing Company for a flight of beer.
Didn't do anything in port, but checking in was very smooth, and customs moved quickly after getting off the boat.
Downtown area is fun to walk around. If you like food, the City Market is worth a look. There are a couple breweries near the dock, too (Big Tide and Picaroon's). The Saint John Ale House has good food and a good beer list. The Loyalist Burial Ground and some of the squares nearby have pretty gardens/landscaping, too.