As a four times a year traveller to DEN I'm delighted that BA at long last has competition. As it happens I've just booked my March 2018 DEN trip (as a BA holiday).
For March 2018 the Y fares Norwegian return of £480 is a lot less than BA's £750. But I travel WT+ and the Norwegian is £1110 cf BA's £975. OK you get much more legroom on Norwegian than BA but it's interesting to see they're not trying to compete with BA on price. From my experience BA regularly fill their WT+ cabin on this route so it will be interesting to see if BA are facing any real competition in the premium cabin.
BA's proposition remains stronger for me (especially since I have a domestic leg) - I pay less up front, get lounge access (and DEN lounge has good food), FastTrack security and by paying and other spending with Amex get enough Avios to get an upgrade to CW (hence the year ahead). It brings home to me just how important BAEC is to BA for retaining existing pax. The occasional upgrade also helps.