We have flown multiple times with Norwegian, its basic but fine, we will definitely carry on using them. If you fly from London they fly Dreamliners, I've never done that. From some or all regional airports they fly 737s which feel quite small. The food, in my experience, is awful even by airline standards. If we need a checked bag we buy the food and bag package but refuse the food when it comes, and take our own. Buying the food does give you some drinks, otherwise you have to buy everything, including water. Take plenty of water on board with you.
The entertainment is also poor on the 737s (I'm not sure if its better on the Dreamliners). It relies on you having a device and logging on to their system which is flaky and the choice of films etc is very limited (you may have to pay, I can't remember). There are no sockets to charge devices either. There is nothing like the seatback systems you get on other longhaul flights. There is Wifi but it is unreliable and usually messes up the settings on my tablet and phone. There are screens on the bulkhead and overhead playing random programmes, I came back from Boston watching a documentary about building a tractor once. They don't dim these screens overnight so bring an eye mask. If you are unlucky you will get the noisy crew who spend the night yelling at each other in the gallery - bring earplugs!
The seats are comfortable enough (but I have short legs and never have any trouble on planes).
Overall for the price its a good deal, just don't expect a full service flight, it is easyjet but longhaul. The airports they fly to are usually small and some distance from the city they are badged as, so the "Boston" flight goes to Providence RI, an hour away on the train. The bonus is that getting through immigration is much nicer than at the big entry points.