Here is a link to Pluto4President's summary of information about National's Emerald Aisle:
http://www.disboards.com/forums/showthread.php?s=&threadid=2608.
To pick up a car, you go to the EA. This is clearly marked at MCO. You will find intermediate and full size cars there. There might also be minivans, SUVs, pick-up trucks, and convertibles. You can choose from what is available. Your reservation will be for intermediate or full size car (frequently the rate is the same), but you can choose a higher class car if one is on the aisle. This is primarily a matter of timing and good fortune. If you reservation is for full size and none are available, you usually can pick from the full size aisle rather than settling for an intermediate.
Drive to the exit where you hand over the hang tag found in the car, your EC card, your drivers license (sometimes you are asked for this, sometimes not; I don't know why since the EC membership information includes it), any discount coupons to be applied, etc. I usually hand over my Disney Club card and credit card as well (although they should have the credit card information already on file with my EC reservation). About a minute later, you should be on your way. No waiting in line at the counter, so the entire process can be completed in about 5-10 minutes from the time you walk into the garage until you drive away (depending on how long it takes to pick out a car; I always like to check the entire aisle before making a selection but my DW would be just as happy to load the first car we come to and go

).
Returning the car is much the same whether or not you use EA. At MCO (and everywhere else I've personally used National), you drive into a return lane where you are met by an attendant carrying a portable scanner/computer/receipt printer. The attendant enters the rental information, mileage, fuel level, etc. and verifies that you still want to charge the rental to the credit card used to secure the rental. A receipt is printed on the spot. Usually, this is for the correct amount (I always keep my printed confirmation from the reservation at the National web site in case there is a difference). One time I had to go to the counter to get the charges reduced, but that was a result of my picking the car up many hours after the scheduled time so the rate reverted to a daily rental rather than the weekly rate originally reserved. As an EC member, you now have the option of having National refuel the car rather than returning it with a full tank. They claim to charge the average prevailing rate for the area. I used this service for the first time a couple of weeks ago, and the price per gallon appeared to be competitive to what I saw at the stations near the airport (this was in Las Vegas). I felt the fuel refill charges were quite fair and enjoyed not having to refuel close enough to the airport for the gauge to still register full when I dropped off the car. Dropping off the car can be quick or slow depending on the number of cars being processed when you arrive. I've never been in a line longer than 2 cars, and have frequently been through the process so fast that the receipt was printed and in my hands before I had the bags removed from the trunk. When this happens, I have to be careful to make sure I don't leave anything behind in the back seat.
Ralph