I used the US helicopter one once - it just wasn't efficient enough. The whole point of the service was that it was 10? 15? minutes to JFK, but you had to clear security at the helipad once, adn then (as you say) it was only for Delta's terminal, so for anything else you wanted to do, you had to get back terminal-side, take the airtrain around to the terminal you wanted, clear security again and find your gate.

Depending on how close you were to the heliport to start out with, say favorably it's a 10 minute walk/cab ride to the helipad, 15 minutes for security and waiting for the helicopter, 10-15 minute flight, 10 minutes to get back airside, 5-10 minutes to the right terminal, reclear security, etc. and it's almost an hour or more in total.

From most places in midtown or lower, it's vastly more convenient to have a black car pick you up and drop you off right in front of your terminal in an hour or so, depending on traffic, while you read, do work, or finish a conference call. You just get in the car, and get out at the other side, clear security once, and go to your gate.

The only time I used it was when I had a meeting a few minutes away from the helipad, I had an international flight I had to make, and traffic was all screwed up due to some major accident, google maps saying it would be 2 hours to get to the airport.