I have cruised with MSC before on Lirica and enjoyed it a lot. This cruise on Opera was just as good if not better in many ways.
Firstly, I really do have to wonder why some people go on cruises because based on their reviews it is just an excuse to complain. The only advise I will offer is book a cruise and make up your own mind because some of the negative reviews are total rubbish.
Don't book a cruise with MSC if you don't like mixing with other Europeans, particularly Spanish, Germans and Italians. Some 90% of the passengers on both of the cruises I have done are European (non British).
A very good sized cabin and very comfortable. Biggest bath I have seen in any cruise ship. Our cabin also had a walk-in wardrobe. Cabins are well maintained and cleaned 2 times daily by very helpful staff. It may be worthwhile taking a travel kettle as cabins do not have them.