A few words about Ezeiza:
One of my guests (who decided to make her own way from Ezeiza - I usually book with Taxi Ezeiza for guests) recently took a taxi from Ezeiza, one of the hustler taxi guys in the arrivals hall not affiliated with any stand or company. He took her out into the middle of nowhere, outside of Capital, and demanded all her money and wanted to take her to an ATM. She had $200 USD on her and gave that to him. He then drove her into town and dropped her off in the middle of the Microcentro at 10pm, nowhere near her destination. DO NOT RISK IT.
In my experience, Taxi Ezeiza are the best of a bad bunch for overseas visitors who want to book in advance. The people in the office are often rude and incompetent, but at least it seems like a safe option. I have been told that the guy who takes people from their stand to the car holds out his hand and says "tip" in English, which is obviously inappropriate, but I suppose one isn't obligated to give him a tip.
Another option, if you live here and have a local cell phone, is to call RadioTaxi Premium - there are often drivers waiting in the parking lot for a return fare into the city. I did that once, and it was easy - they send you a confirmation text with the driver's car number and license plate information so you know it's the same car you called for. I waited perhaps five minutes, and I believe he charged me less than Taxi Ezeiza would have - I don't remember.
On another note, another guest changed money at the Banco de La Nacíon in Ezeiza arrivals and was given 300 pesos in fake bills. Just enough (I'm sure the thief thought) for it to not be worth their while to take a taxi back and complain. I contacted the airport police and then drafted a letter in Spanish explaining what had happened (with dates, times etc) for them to give to the airport police on their way out of the country.