Getting to and from New York's airports is such a hassle. Supershuttle is one of the best options I've found in terms of the price/convenience ratio, but there are a few things you must keep in mind.
First, lie about when your flight time is, especially if you have an international flight. Sometimes they pick you up way too late to arrive at the airport in a comfortable amount of time. Say your flight is an hour earlier than it is.
Returning from the airport, be prepared for a very, very long journey. It once took a friend of mine over two hours to reach me.