Norwegian Air - NY

Here's what I know about NAI and what to consider:

The aircraft are pretty new and the service is on a par with easyjet as is the seat pitch. They are better than Ryanair but like all loco's you pay for everything including seat allocation and bags on the long haul services so factor that in. There are no in flight entertainment screens so bring films on your tablet. Some aircraft have wifi.
The NY service goes to Stewart which is some way away from NYC. There is a bus but apparently it's as quick (and cheaper) to go into the local town and get a train.
there are no interline connections and few regional services from Stewart.
The service isn't daily. If it gets cancelled you might wait days for another service into BFS. They may offer you DUB/SNN or have been known to send people to EDI instead. This is where the lack of interline partners starts to bite as coming into to winter you can guarantee weather delays or cancellations.
These aircraft are small by Atlantic standards and the ocean weather can be pretty rough in winter. If you don't fly well you might prefer choosing a wide body service.
At present the loads on both routes from BFS aren't great. Good if you want extra space but increases the risk of cancellations.
Like Ryanair, Norwegian have some crewing problems but not as severe as Mr O'leary's issues.
They are losing a lot of money (shed loads of it) but are seemingly well backed by The Norwegian Oil industry and alike.

Personally, if the return is around £200-250 including bags and seat allocation then I'd give it a go. More than that or if your time is precious and you really need back around a given date then I'd be looking at DUB.

I used Norwegian's service in October. For £263 per person return including a meal, luggage and seats in the exit row I thought it was very good value.

The service itself went without a hitch. We took off on time and arrived 45 minutes early. In-flight, if you haven't ordered a meal there is very little to purchase. It is not like short-haul low cost carriers that sell a lot of food options from the trolley - I don't think there were any hot options to purchase. They also didn't sell any duty-free products. Therefore I think it is a good idea to pre-order food. The meal was tasty enough and included wine/beer and tea/coffee. Otherwise, bring a meal-deal from the terminal or something.

The airport is small, the terminal is about the size of Belfast City Airport and our flight was the only one arriving at that time so there was very little delay in getting bags. There are machines you can use to speed up getting through customs and I would recommend using those while you wait for your bags - it lets you skip the immigration line.

The coach to Manhattan is timed to match the flight so we still had to wait about 50 minutes for the coach to arrive. The trip into the city was smooth and there weren't any traffic issues. The coach leaves you at 41st Street / 8th Avenue and you get picked up at the same spot for the return. The coach took 90 minutes.

On our return, we chose to get the coach scheduled for 5 pm. We arrived about 4.40 pm and it was already waiting. It left dead-on time and there were only 27 people on it so there was no issue with storing luggage or getting a seat. The rush-hour traffic delayed us a bit so the journey to the airport took 2 hours.

Stewart International Airport really doesn't have much in it. There is a cafe after security but it was very busy because there were 3 Norwegian flights all leaving within an hour of each other. Aside from that - it was grand. I did notice that the Dublin and Edinburgh flights were both full and the gate staff made announcements inviting people to have their luggage put into the hold for free. The Belfast flight was only about 2/3 full. Many people were able to get a whole row to themselves to stretch out and doze on the way home.

The new Boeing 737 Max aircraft do not have WiFi yet and there are no seat-back screens so bring your own entertainment. (I think WiFi is "coming soon").

I'd definitely consider this again for a trip to New York, if the price worked out well. If I was to go from Dublin, I'd have to get a coach or drive down there and back anyway so that road journey doesn't put me off. The lack of pre-clearance, compared to Dublin wasn't really an issue either since the airport is so small.

Downsides, as noted by others above are all to do with "IROPS": what happens if something goes wrong. You may be stranded in America for longer than you'd hope if there were cancellations. You'd still be eligible for EU compensation under the correct circumstances as Norwegian UK is the operator but they just don't have multiple flights a day the way Aerlingus and others do in Dublin.

In conclusion to this LONG post (sorry) - I'd recommend it if the money was right. I'll definitely be keeping an eye on this option in the future.