I've sailed almost all of the major cruise lines, but this was my first time on NCL. I was not impressed. The food was HORRIBLE (dining room + buffet, all meals of the day), which was extremely disappointing because I have had such amazing food on other lines. The service was sub-par---our room steward continually would take our beach towels and not replace them--then would be nowhere to be found to ask for new ones (while everyone else's steward was easily accessible), dining room service was super-slow and would forget things. The room was by far the smallest I have ever stayed in (9165). I also did not think the areas on the ship were anything great (bars, theatres, etc). There is a $15 corkage fee for wine brought on---I've never seen this before, generally I've been allowed 1 bottle free per traveler.
Now for the good aspects: Price was right (~$300/pp with tax for 3 nights), booking bonuses (unlimited beverage package, free gratuities, etc--but these seem to be offered everywhere now), the private island (Great Stirrup Cay) was the best Caribbean beach I have ever experienced. Very easy embarking/disembarking process (we carried on our luggage each way, which saved lots of time). There was no formal night (which was a plus and a minus....I kind of like the formality and tradition of this on a cruise). I do not regret my vacation on the Sky at all, however I will not be booking with NCL in the future.