I share most of the views about long term stability of Norwegian and agree with everyone on the need for a plan B. I have done quite a few flights with them with mostly good results.
Did fly them SFO-LGW in Premium this week with a close in booking @ 550 GBP or so. From flightaware this route has been running like clockwork.
Swissport are the handling agents for SFO and the checkin was better than United would have been even with status. No moaning about my obviously overweight carry on on the scale and a PRE BP, so they do pick up Nexus/GE. No lounge on the cheap Premium tickets any more. A huge improvement over OAK though!
Very attentive crew and a fine onboard experience. You do not fly Norwegian for the dining experience but it is better food than United Economy Plus .. but not by much! Wifi (free I think) did not work but it never does on United either. Nice to see that they added it and crew seemed a bit surprised it was busted.
I have not flown United in the new Premium Economy but for sure the seats are better than the equivalent BA WT+ 787 product, which I have done.
For those who have not done LGW, the car rental there is instant in terminal including the cars, so a huge win over LHR.
I have not flown them economy except for intra-Europe but the config on the 787 in economy is the same as everyone else. For anyone who does make sure you fully meet the bag limits or pay up. Expect to pay for everything on board.
My advice would be to enjoy it while it lasts, great premium product for the money! No self control by management and they are stuck with the aircraft, so have to fly them anyway. From the fare matrix, it looks like they are flying LGW-SEA all this month with horrible loads. No interline, so they cannot cancel and reroute the way the legacy carriers would. I am not in the airline business but they really need to focus on the core routes and shrink down if they want to make it through next winter!