I just flew Norwegian on a premium ticket, from Gatwick to Boston. I had a mixed experience. I'd purchased the base fare through my Chase card, using points, and then upgraded to premium with dollars. I might have been happier flying in economy, because the extra money I paid wasn't worth it to me.
(1) The big perk for going premium had been lounge access, but the available lounge at Gatwick was crowded, provided only one tiny meal, and due to not being able to check in until three hours before the flight, I didn't get to utilize it very much. Three hours may sound like a lot, but it was actually a much shorter time before I had to head to the boarding gate, and it took a while to get through security, locate the lounge, and gain access.
(2) I had planned to check my roller, since this was my flight home, but they wouldn't let me do so until two hours before the flight. There seemed to be a glitch with the luggage handling system that day. So I kept the roller and lugged it through the airport.
(3) The food tasted really bad to me, and I didn't end up eating any beyond a few bites.
(4) I couldn't figure out what was so great about the entertainment system, though in all fairness it may have been broken. There was some sort of apology made over the intercom as we landed. I saw only a very limited number of movies and TV shows available.
However, none of these drawbacks were so decisive I'd never fly them again, but I'd probably get a very low-priced ticket, bringing my own food and avoiding buying any of the perks I didn't end up utilizing. I would also buy direct from them, because purchasing through a third party had a lot of drawbacks.