When things go well with Norwegian it's a fantastic airline. But I've witnessed some absolutely awful experiences others have had - fights delayed 11 hours, some even more than that, leaving everyone stranded in the airport. Other times they've contracted out flights to a no-name airline that uses ancient planes with no in flight entertainment.
I'm happy to take Norwegian when I'm travelling by myself, as I'm content to take the risk. But if I'm travelling with family I'll usually avoid them.