We've travelled 12 times or so on Celebrity Cruises, but this was our first experience of NCL. The first and most obvious difference is the size of the ships: in terms of passenger capacity, almost twice as many as some the Celebrity ships. In some parts of the ship, you are scarcely aware that you are at sea.
The route was Barcelona to Barcelona, with stops in Cagliari, Palermo, Naples, Rome, and Livorno.
Good things were:
The cabin was about what I expected. Adequately comfortable. Sufficient storage.