We have been on numerous cruises, but never NCL. We selected the Gem based on the itinerary. The ship itself was very good. We enjoyed the layout of the ship. The staterooms were good and we really liked the fact that it had a much larger shower that we have experienced on all our other cruises. The ship is extremely clean and we enjoyed the "day beds" by the swimming pool.
The food was average - I think that was because of their "Free Styling" cruise program where they have numerous speciality restaurants which you pay an extra fee to utilize. As such, I think that the food selections in the main dining room (which is included in the basic price) were limited and didn't have a lot of imagination. If you wanted many of the dishes that we have had on other cruises, you need to go to the speciality restaurants (where the costs is from $10 - $25 p/p). We did try the mexican restaurant and it was very good. Drinks were also expensive (beer $7 bottle).
The entertainment was probably the worst that we have ever had. They had one singing and dancing show that was very good and one dancing show that was good. They had two show - a ventriloquist and magic show that were absolutely terrible.
The other issue we had was that they provided no information on the ports of call (other than a very simplistic map). All cruiselines push their excursions, but they have all also provided some basic information for each port of call (how to get around, where beaches were, some general shop information, etc) - to the best of our knowledge, NCL didn't provide any of this. Also, while we spend very little time in the room, the TV channel selection was terrible - other than their 24/7 info commercial on their excursions, the only other channels were BBC, Euro Sport and the entertainment channel E. If you wanted a movie, it cost you $10 per day.
We had a good time and thoroughly enjoyed ourselves and the ports of call, but in the future, if we have a choice, I don't think that we will cruise with NCL again.