I have heard that the best ATM's to use as those with a sign saying "LINK" in a green circle. At least in January 2008, I had no problems with those, getting as much as 1150 pesos. (Deliberably chose an odd amount to get some small bills, too.) For me, I never can exceed the equivalent of $400 USD per day because that is what my American bank limits me to.

For example, there is a Banco de Nacion "LINK" ATM right inside El Calafate's airport. That worked fine there and elsewhere.