I just like to post my experiences with ExecuCar. I landed at JFK on-time and was quick through immagration and baggage claim. I prebooked a sedan meet and greet service. However there was no driver there and after a number of frantic phone calls I had to wait an hour for the driver to come and pick me up
This was totally unacceptable and I did not get a refund.
However their service in DC from my hotel to IAD was on-time.