My girlfriend (now fiance :-))and I went on the MSC Poesia from November 14-19, 2011 from Fort Lauderdale to Costa Maya and Cozumel. I'd probably rank our cruise about 6/10... here is why:
The boarding process was chaotic. We went through security in the port, then were told to sit in a "holding pen" until the security personnel decided we should be permitted to enter the queue for a check-in agent. It only lasted about 15 minutes, but it seemed arbitrary and the order in which people were chosen did not seem completely chronological.
We then boarded the ship. It is a beautiful ship. Barely a speck of dust, let alone any dirt. Our cabin balcony had a great view. However, once we began wandering around the ship, we had a hard time avoiding second-hand smoke. On our previous Carnival cruise, smoking was permitted on one side of the Lido (pool) deck and was not permitted in any of the public indoor areas, except the casino. MSC permitted smoking on one side of the pool deck, as well as a few select areas indoors (incl. at least one or two bars/lounges, as well as the casino). I expected smoking in the casino, but because of the great latitude of indoor areas where it was permitted, the smell permeated most indoor areas of the ship (even "non-smoking lounges) with the exception of the main theater and the dining room. Even on the "non-smoking" side of the pool deck, people were occasionally lighting up. To their credit, the staff did talk to someone for us at one point when they were smoking with their feet in the pool.
We found out right before boarding that our cruise was part of "shiprocked". I would have loved a 5 day rock concert/cruise, but my girlfriend just wanted to relax and see the usual cruise shows. However, you needed a special pass for access to the main concerts... and, for example, we paid $300 each for our cat. 10 balcony - it appeared that the same cabin would have cost $1349 with shiprocked access. In any event, there were no non-shiprocked shows on the pool deck (only a few dances). And, the main theater only had one show per night at 7:45. So, if you wanted to see any shows on the cruise, you needed to eat at the first seating of 5:45 every night or eat at the buffet. Carnival had entertainment in the theater the entire evening, so it did not matter when you ate (not to mention that Carnival offered open-dining). Basically, there was the one show at 7:45 until approx. 8:30 (all of the shows were really good, though), then 2 mini-shows in the "Zebra Lounge", which was nice, but not an actual theater. This was necessary so that they could have a shiprocked show every night at 10:30 or 11 in the main theater which only laminate holders had access to. This wouldn't have bothered me so much, but we were not told of any special events prior to the cruise which would use up a lot of the public spaces and restrict entertainment options. Carnival had multiple options at moment of any given night. Carnival described itself as the "Fun Ship". MSC may not be the "boring ship", but isn't that far off. One entertainment option at any one time (other than bars) is not enough.
The food was really good. I was surprised because a number of reviewers had complained about the food. I had the mozzarella caprese as an appetizer almost every night and really enjoyed it. Also, I had a rib-eye steak in the main dining room one night. No food related complaints.
I cannot comment on the excursions offered by MSC as we booked locally in Costa Maya and just took a cab to the beach in Cozumel. Our "Mayan Experience" through www.thenativechoice.com was really good. Our tour guide, Ivan, was very knowledgeable and the Mayan lunch was delicious. I'd recommend this tour without hesitation.
Overall, although I am critical of certain things, the cruise was a lot of fun, especially for the price we paid ($334 incl. taxes/fees). However, MSC doesn't compare to Carnival in terms of entertainment quantity. Also, I understand the MSC is an Italian cruise line and Europeans smoke more than Americans, but MSC should adjust accordingly while sailing in the Americas.