We cruised on the Norwegian Gem on NCLs last greek islands cruise of 2010 before the ship heads back stateside for the winter. We were travelling as a party of 6 across 3 cabins (2 adults with a 6 year old child in our cabin).
Overall we were very impressed with the experience, our previous cruise was on the QM2 so the Gem had quite a lot to live up to. Despite this we were very favourably impressed by the experience, all of us had a lovely time and relaxed completely. Our 6 year old son especially enjoyed the kids club, which is staffed by an excellent team who ensured he always had a superb time. He also had a great time in the pools / slides on deck 12, which is a great area (but can be a little noisy at times if you don't want to listen to the music).
Our only niggle would be the ever so slightly 'in your face' NCL policy of trying to make a buck off you where ever they can - in most cases we ignored this but it can lead to costs rising on this type of cruise if you are not careful. An example is buying a beer by the pool - you may want them one at a time, but they assume you want the 'buy 5 get one free' deal and try very hard to make you take them up on it.
Well designed cabin
Good food and plenty of venues
Nice range of facilities
Excellent for children