Don't pay for anything here with credit cards or get money out of the ATMs, unless you like to lose money. The official rate is held down artificially by the government, and everything will be quite expensive at the official rate due to somewhere around 40% annual inflation and how far down the official rate is being held. You are talking right now roughly 9.1 official vs 14.3 on the "blue" market (which is really a black market).
Don't change at a bank or an official cambio place (if it says "cambio" - run
places for the blue rate don't look like a cambio ), or at the airport or your hotel. If you need some pesos to pay your ride into town and have a little spending money until you can get to a blue market changer, change a little bit in the airport maybe, but make one of your first stops, once you get checked in, to be a blue market change place. If you can buy some pesos before you come down at your point of departure, get some "pocket change" there. PM me if you'd like a couple of discreet locations where you can change on the black market (depends on where you are staying. I live close by where most tourists come to stay in the city).
As has been mentioned, it is indeed illegal to change on the black market, but everyone does it. Unless you have so much money to burn that you don't mind paying around 35% more for things than you have to, you should bring as much cash as you can for your trip and pay everything you can in pesos. Just be very careful what you do with the dollars. It is a balancing act - if it gets stolen, true that the 35% or so you would save kind of goes away. But if you're in a decent hotel, you should have places to keep it, like in a safe (I'd hope they'd have).
You can also bring less dollars down with you if you are planning a trip to Uruguay (one hour ferry ride across the bay). You can get dollars out of the ATMs there and bring them back with you and an afternoon or so in Colonia, Uruguay wouldn't be terrible. They have limits on how much you can get out of the ATMs per transaction, I believe it's $200 (but you can do it as many times as your bank's limit), but a lot of people who live here make that trip at least on a monthly basis.
When you get here, there are many, many discreet places, as well as open-air places (where the black market sellers of pesos stand outside and say "cambio, Dollars, Real, Euro" all the time - and they are protected by police, so the illegality is quite gray) where tourists, expats and locals alike change money. The only thing uncertain is to know what the price will be - just last month the black market rate was much lower than it is now (around 12.70) and next month it will probably be somewhat higher on average than it is now (most of us hope).
Look here: http://www.ambito.com/ and find the "Dólar (Informal)" to get a rough idea what the black market rate is for the day. It should be around .10 - .20 centavos less than the "Compra" column that you can expect to pay, usually. With the rate floating more now, instead of being fairly fixed, Friday through Monday are the days most likely to have the worst prices - try to change Tuesday, Wednesday or Thursday (although any day is much, much better than the official rate).
It's really not a big deal to change your money on the black market and really, really worth it.
And as another mentioned - the ATMs here in Buenos Aires will give you money (pesos - never dollars). There are some that don't work so well with foreign debit cards at times, and at other times the money in the machines runs out and are waiting to be restocked. But the limits are small and transaction fees high, and you have the official rate to contend with which just makes it all quite unpalatable for a seasoned traveler.
