VPescado did a good job of outlining the somewhat unusual EZE procedures. I'll add the following.

First, despite some earlier warnings, I found EZE check-in to be relatively speedy. I left Centro 3 hours before my 9:15 flight, and that was plenty of time (at least on a Sunday -- I understand the traffic is worse on weekdays). Coach check-in at AA a little before 7 pm was surprisingly speedy (I have no status on AA so I just waited in the regular line). There is a somewhat hidden airport tax booth to the left of the AA counter, which is a better place to pay your $18 than upstairs. The booth takes dollars, pesos and credit cards.

On the dual immigration line, look to see if the left side is shorter. Most travellers instinctively head right!

There seemed to be lots of people ringing up purchases in the duty free store, but I thought the prices there were extremely high. There's also no place to buy a bottle of water or a soda at a normal price, so I'd recommend bringing some fluids with you from BA (they wouldn't be confiscated until you do the final security check at the gate).

If you still have some left over pesos, there is an exchange booth next to Gate 2 that has a plausible exchange rate with no commission.

Finally, searching the internet, I found an English-speaking driver to take me to and from the airport. At $20, he was a bargain compared to current taxi rates. He's also a nice guy with an interesting story. Prompt service both ways. He can get busy, so I'd try to book a week or two in advance.

http://dantesairporttransfers.hazedaze.com/