When we fly from the Lehigh Valley to Disney, we always use Alligiant. Good, friendly, cheap service.
As far as flying into Sanford, we actually prefer it. No, you don't get to use the ME, but the airport is nice and small, easy to navigate, much like ABE\Lehigh Valley, and frankly, it makes it worth the extra few dollars to get a ride to Disney, or rent a car.
My only complaint is sometime late last year, they changed their flight times. They used to leave Allentown at like 7am, now it seems it is much later, and also, an earlier return from Sanford. Kind of cuts your first and last days short. (It's actually one of the reasons, along with some others, that we drove for the first time this year).
Other than that, we have zero complaints with Allegiant.