To get Pesos, ATM is best, withdraw the largest amount you can, if you have bank fees. No fees are charged by the banks in Buenos Aires, at least not by four-five banks I used. In 2007, one could withdraw AR$1500 per transaction, but in 2008 most ATMs now limit each transaction to AR$320. Multiple ATM withdrawals are possible, but this could ring up large ATM fees from your local bank, so find a Credit Union or other such US bank that does not charge for ATM withdrawals. Read up on your bank policies and charges before you leave on your bank's web site. The 2007 US$ cash exchange rate at the bank was also good - AR$3.09 to US$1. The ATM gave a better rate - AR$3.10, and this was exactly the same rate as listed at the OANDA, The Currency Site for that day. In fact, my bank listed a 2% foreign exchange conversion surcharge, but as far as I could tell, this was not being charged or they were starting with a more favorable rate than the one listed at OANDA or in the banks in Buenos Aires.
Small notes and coins can be a problem, so it is always a good idea to have coins and small currency notes. If you are exchanging US$, ask for a mix when you exchange money at the counter at the bank. Mix of 10-20 AR$1 coins, AR$5, AR$20, AR$50 notes is useful, ATMs mainly provide AR$100 notes, so it is better to withdraw amounts ending in AR$90 to get a mix of the denominations.
On the matter of coins (monedas), in 2008 the shortage has turned into an epidemic - every small shop, grocery store, laundry carries a sign no hay monedas. Banks are supposed to give out the AR$1 coins to people, but the first bank I tried only give me only 14. Second bank I tried later on gave me 20 in exchange for a AR$20 paper bill. So, one could go once a week to a bank to get coins, if needed. Coins are the only way to pay for the colectivo. But if not riding the bus (many people may find the system of buses too complicated in Bs As!), then one can generally get by without too many coins - in restaurants, round up the tips to AR$2, and same works for taxis. And in the mid to expensive restaurants - well, this issue is not relevant since you will be dealing in much larger amounts!

Credit Cards are also a good choice, but expect a minimum of 3% surcharge, and could be as high as 5%. Therefore, if the ATM rate is AR$3.10, expect to get AR$3.00 or less for each US$. Some related information is also at the Montreal money page.