Goofy's Mum - I seem to remember that we got our Emerald Aisle card through Biztravel.com for free, but unfortunately I'm not positive. At some later point we changed it to a Magic Kingdom Club (now Disney Club) version of the card, since that seemed to offer a better discount. We had previously rented through off-site car companies which required finding a shuttle bus, waiting for it, loading luggage, and then competing with about 100 other impatient travellers to get a car.
Since National is located in the Orlando airport, we just rented 2 luggage carts ($1 American each - don't forget to take a few small bills with you) and walked to the lot. We had made a reservation through National's website, and (at least at that time) if you rented a mid size car or higher you were entitled to choose from the Emerald Aisle (EA), and were guaranteed to get at least the car category that you had booked. We followed the signs to the Emerald Aisle, loaded everything into a red Chevy Blazer (thinking it would be easier to spot in a parking lot), turned the key in the ignition and drove to the check-out. We spent maybe 1 minute with the parking attendant, who recorded the car # and reservation# and we were on our way! It was just so fast and so easy. There were some Chevy Blazers, a Jimmy, a pick-up truck and a PT cruiser to choose from, as well as several mid and full sized cars. I didn't even let the kids see the PT cruiser, because I was worried that our luggage wouldn't fit. I'm afraid that I don't believe in doing laundry while I'm on vacation, and it shows!