Flying in and out of ISP is a no brainer. LaGuardia and Kennedy (excluding JetBlue terminal) are absolute disasters.
I live in Nassau County. JFK is close, LGA is next and ISP is the furthest of the three local airports. I'll take ISP almost every time. In the event that I can't do ISP, I'm more likely to drive up to White Plains than to go to either LGA or JFK.
There's just something about flying out of a small airport that can't be replicated from the larger ones.