I fly out of Islip and can attest that GA really rules the airport. The airline service is really minimal with only Southwest and very few USair reginal service. That being said, the airport is not really crazy busy. I never wait more than 5 minutes to take off and landing is a non issue. It's a great airport so I'm happy they are doing good bussiness!
The landing fees, as far as I know, are for aircraft not based at Islip. Their motivations are many but at least amongst the pilots from where I rent from, the sentiment is positive because so many planes use MacArthur airport for practicing that it can get crowded in the pattern. Even planes from Farmingdale, literally like 15 miles away will come to Islip to practice in the pattern because their airport is so crowded. I'll be curious to see how it works.