They have been flying from LAX to Gatwick for more than a year now. I checked this route for some random dates in May and the price is $703, not bad. What disappoints me is checking this route for July and August dates. Their very lowest prices for July are only $180 less than several legacy carriers and in August they are less than $100 different. It will be interesting to watch this airline and its growing US market to see if they are creating the competition or inching up their prices for peak summer travel months. If the latter is the case, I'd book with a major airline if the price wasn't much different because there are more flights per day and partner airlines for the legacy carriers, something to fall back on if a flight gets cancelled.
Our kids (2 adults and 2 children) flew on Norwegian from LAX to Gatwick last summer. Sure enough, there was a flight cancellation that caused them to have to fly 2 days earlier or 3 days later. They picked the only option that could work for them because of travel commitments going forward from London.