The wife and kids just used Southeast out of Newburgh last week. They had a good trip and were pleased with the airline. The return flight was delayed by an hour, but that could happen with any airline.
Southeast airlines regular fare to Sanford appears to be about $213 or so - which is very good compared to the Delta rates out of Newburgh for a connecting flight to Orlando. I am going to guess they do have sales now and again, but even with their current 'A fare to remember' promotion the price is coming up $213.
You can join their Club 59 frequent flier program. For an individual you pay $99, a family of 4 is $199, and you can fly for $59 each way. Apparently there are no blackout dates. I'm not sure if they limit seats. I also don't know if the membership fee is one time or annual.
It is unfortunate that flights out of Newburgh are as expensive as they are. When Newburgh first opened for commercial traffic we flew out of there frequently and got pretty good prices. They built a nice new terminal, went to a private (as opposed to state) operator and I think there are less flights out of Newburgh now than there were when they first opened. They make all kinds of claims that if you book 14 day advance the fares are as cheap as the major NY airports - but that just isn't true. A flight out of Newburgh might be worth a little more for the convenience and cheap, easy parking, but for $100 a ticket more for a connecting flight it just doesn't make sense. Southeast does provide a direct flight that is not that much more than the NY airports, but the flight schedule is rather limited.
All in all I don't think that Stewart has become what they envisioned it would - that is too bad. While Stewart is less than a half hour from us, we use Albany, Bradley, LaGuardia, or Kennedy to get better priced direct flights that fit our schedule.