I flew Norwegian nonstop from FCO to LAX on June 20, 2018, in economy class. The flight was very nearly on time, and was uneventful. It was full, mainly Americans. On-board service was routine, although I did see a couple of very inexperienced crew members who looked as though it was their first flight as stewardesses. Disconcertingly, the pilot announced, TWICE, that the flight was from FCO to SFO, until he was corrected by a flight attendant (perhaps he did not get much sleep). I did not pay extra for the awful meal service or for an assigned seat. Observing the meal, I'm happy that I avoided paying $45 for what appeared to be a most unappetizing entrée; indeed, the woman seated next to me did not eat any part of her meal service, and offered it to me (which I graciously declined)! As for economy class seats (3-3-3), they are all equally bad! I was seated by the corridor, so I could get up without bothering the passengers next to me (in that sense alone, aisle seats are better). The seat was hard and did not recline adequately for a 12-hour flight. Leg room was adequate, but knee and seat room very tight. My major criticism is in the lack of a good recline, which made the flight somewhat uncomfortable.
My recommendations:
If you can get a super-cheap economy class seat, go for it! Paying $500 round trip instead of $1200+ on Lufthansa or another legacy carrier is well worth it. However, if you're paying about the same fare as on the major carriers, I'd go with the latter. Skip the extra-cost meal service. Skip the assigned seating (unless you must have a window or an aisle seat). If you can travel with only 10 kilos of carry-on luggage, save the $45 surcharge for checked luggage (maximum 20 kilos, not 23 kilos). Remember to bring your own earphones, pillow and blanket (or overcoat). Bring your own food and a large bottle of water. I don't think that the weight of the food and water will cut into your 10-kilo on-board allowance (use a separate plastic bag).