I have used them a few times for Oslo and once for Tromso and rate them amongst the best.
I enjoy going to Scandinavia, whatever country it is, as they all seem to share a commen high level of products and service in most industries. Of course, the cost of living is a bit more but the higher quality and professionalism is fairly consistant. Norwegian Air manage to offer a low cost option without losing this quality.
With reference to the poster who rang the airline and did not like the response, I would like to point out that Scandinavians generally are efficient and to the point. When added to a slight language barrier, this can come across as a little brusque. (I understand this more because as part of my job, I cold call various financial companies in that area of the world and don't speak anything but English).
It's great that they now have a Gatwick hub as next Spring, I intend to fly to Split, Croatia and Norwegian Air will be my first choice.