Part of a larger group of friends, we were a family of four split between a Haven-class suite and an inside cabin, so we experienced the gamut of what NCL has to offer on these brand-new ships.
For this review, I'm holding the Haven class experience to what you might get on Crystal or Regent or Queen Mary Princess class, because the Haven class pushes the fares closer to luxury.
First the highs:
As many have noted, the ship is beautiful throughout, though the exterior is noticeably cheap looking.
Entertainment was excellent, other than the piano player, who couldn't sing an Elton John tune to save his life. Legally Blonde, Burn the Floor, Howling Pianos, and Illusionarium were all top notch and lots of fun, though the meal at Illusionarium was bad even for banquet food. The smaller bars, game shows, movies, and dance contests were a lot of fun, too, and helped pass the time on the sea days.
The speciality restaurants were a pleasant surprise. The Zakarian restaurant, Ocean Blue, was excellent, despite a slightly tame menu as is the case with his US restaurants. Get a Waterfront table and you'll have a fantastic evening. Teppanyaki is exactly what it should be (it serves the worst green tea ice cream I've ever had, however). The Italian restaurant is good as well. I highly recommend the rolls at the sushi bar, and the Shanghai noodle joint is worth visiting as well.
Spa and fitness would be hard to beat.
Service is mostly excellent, though the crew is clearly overworked.
The Haven really changes the experience, though if all you want is a quiet space, I think it would be more economical to spring for the Thermal Spa and hang out there. The Haven restaurant is EXCELLENT, saving you from the ship's uniformly mediocre breakfast and lunch buffet offerings. The concierge and butler service are vastly overhyped (we didn't even meet our butler), but they do make for a smooth experience. If anything, some of The Haven amenities make for an awkward experience, like the early/ priority seating at shows and the elevator that is somewhat ridiculously dedicated to Haven guests.
Embarkation and disembarkation were fine. No welcome drink of any kind, though.
The water slides and rope course really add a nice touch.
Preteen and teen programs were excellent. Not Disney, but very good.
And now the lows:
Dealing with NCL is a nightmare. I urge you to go through a travel agent and to avoid their "cruise consultants" at all costs. The one we had was just dreadful, gave lots of misinformation, and botched the entire reservation, requiring calls to corporate headquarters.
Complimentary dining is uniformly mediocre if you're any kind of foodie or even just like nice restaurants. The main buffet is revolting. The three main dining rooms have incoherent cuisines and bland food. The pub has great atmosphere (and is open 24/7) but lousy food--doesn't even compare well to a good sports bar.
For anyone who has traveled a luxury line, the nonstop charges (and sales jobs) are annoying, but not enough to ruin your trip.
Overall, I'd probably take the Haven fare and spring for a luxury line next time, or perhaps Disney or Celebrity, but if we went with friends or family again, I'd be more than happy to travel NCL.