It's been recommended in the past that visitors have taxis called instead of hailing them off the street.

At any rate, you may consider taking the Manuel Tienda Leon remise (used to be after you exit the customs area but before you emerge into the meet 'n greet area) which is a private car operated by a large transportation company. MTL also operates a shuttle bus to the city if you want to save a bit (ARS 35 vs. ARS 105).

http://www.tiendaleon.com/home/home.asp