Been to America three times in the last four years and I think we waited at customs longer than it actually took for us to fly there!
I've flown into the States through Atlanta, Chicago and Seattle within the last year and never waited more than 15 minutes at Customs!
It's a shame Norwegian are pulling some routes - I flew back from Seattle to London last week with them for £164.90. I'm back in Seattle next month but they've pulled that route for the winter meaning that I have to go up to Vancouver now to get home.
I think what Norwegian have done is just buy up all of the routes they could, see which ones are viable and cut the ones that don't work (or at least make them seasonal). Very happy with them as an airline, never have a problem.