Immigration lines are a simple game of chance I'm afraid, as others have said it is sometimes clear and you are through in a matter of minutes and other times you are wondering why so many people are wanting to use the restrooms at the same time

You could book it for an hour after you are due to land and in case of a delay in immigration, simply give them a call to let them know. As tonty says a car usually appears within a matter of minutes.
I have dealt with Dial7 before (through a V-Flyer recommendation - cheers tonty) and they have always been accommodating. Curb side (due to restricted parking at the airport lots) pickup at EWR is always efficient and the service at JFK is good too. I tend to call them on the transport free phone on arrival in the arrivals hall to get directed to my driver or pickup point.
Sometimes the touts will try to con you into thinking they are there for you. Any probs point airport security their way and watch them scamper

If all else fails the taxi lines tend to move quickly and with fixed fare into Manhattan makes another sound option.... ahh so many choices.
P.S - If you are looking to avoid the tolls on the midtown tunnel, ask them to take the route via the Queensboro bridge into the city. ):