I'm surprised people think the fares are cheap, because for the searches I have done they are more expensive than United.
In Sept 2017 I can fly United to Newark for £370 rtn in economy with a meal,IFE, checked luggage etc.
In Sept 2018 I can fly Primera to Newark for £365 rtn with carry on only, no meal, no IFE. If I want checked baggage and a meal it's £465.
Depends when you look. August 2017 vs August 2018, United is £1288, Primera is £505. End of September 17/18. United £537, Primera £421. Earlier in September, United £388, Primera £463 (£364 "low fare").
With the exception of the "low fare", the above prices offer checked luggage and a meal.
Personally speaking, a meal I'm not too fussed about, I can have one in the airport cheap enough, before I get to the airport, or take something with me.
Checked luggage, depends how long I'd be going for. Short trip, I could manage with cabin baggage, longer trip, with the exception of the earlier September option, checking luggage is included, and even adding luggage to the "low fare" option only makes it £26 more expensive than United.
IFE. Whilst using an on-board, built in system is easier, most people these days have a laptop/tablet which you can watch films on, so not a deal breaker.
Yes, in places it may be a little more expensive, but I'd happily take more expensive tickets if it means having an airline flying to 4 N.American destinations, rather than an airline with wavering commitment flying to just 1.
Remember, Primera will most likely be advertising itself as a LCC. People are prepared to go for the most basic fares for those airlines.