I have been on eight cruises and have cruised with Royal Caribbean, Norwegian, Carnival, Costa, and Viking River. All have been great. This MSC cruise, however, would turn anyone against cruising.
I chose this cruise because it began in Buenos Aires and featured two days in Rio de Janeiro, the two places I most wanted to see in South America. What MSC does not tell you is that this ship is operated like a hop on, hop off bus, allowing people to check in and out all along the route. This meant that the ship was in a constant state of upheaval, with passengers who were embarking and disembarking competing with cruisers trying to get on and off the ship for excursions. I don't know what went wrong on the second day in Rio, but those embarking passengers were packed into the terminal for several hours waiting to board.
Other issues with this ship:
It was nearly impossible to get a drink. There were not enough cocktail waitresses in the lounge areas and if you tried to get a drink at the bar the bartender ignored you in favor of filling orders for the waitresses. Also, many of the lounge bars were closed on sea days so if you wanted to relax with a book and drink in one of them you had to wander around looking for an open bar.
The food in the dining room was mediocre and unvaried. Every salad sat in a bath of watery dressing because the lettuce hadn't been dried properly after rinsing.
Two elevators were broken during the entire cruise.
The buffet area was always crowded; my guess is the short-term passengers were determined to make the most of their brief stay by eating as much as humanly possible.
There were lovely lounges on one deck -- with all the views completely blocked by lifeboats.
In short, I would warn anyone off this particular cruise and, personally, I'd be afraid to trust MSC again.