We did a cruise on the Mercury in March of 2002, We were informed there was a propulsion problem and the ship could not make the fully itinerary. They made alternate stops and we were given a $100 per cabin shipboard credit.
There was also a pronounced vibration - felt especially in the aft section of the ship. This coincides with their claim of a propulsion problem. They never did specify what the problem was.
In the dining room - the vibration was extremely annoying - and I had our table moved as far from the center of the room as possible. I never did enjoy dinner until the last evening when they got much of vibration and propulsion problems under control.
But, I would not hesitate to rebook on the Mercury if the timing and price worked out for me. I like Celebrity as a whole - and I like their ships and service.