When we researched our first trip we were concerned about relying on ATM's because of some of the issues others had run into. About a month before we left we contacted our banks to let them know we would be traveling so they wouldn't put a hold on our accounts when they saw a foreign transaction. We brought enough cash to survive on should we run into banking issues. We also brought plenty of US 1's and 5's for tipping (We were told small JA bills can be hard to come by and it was often the case). We exchanged a very small amount at the airport (bad exchange rate) so we would have some JA on us when we landed. We exchanged the rest of our cash at a cambio the next day. We both used 2 different ATM's with 2 different bank cards without any problems. We brought a small amount of JA home for next trip. Both banks charged us a $5 foreign transaction fee and one of the banks charged an additional fee based one a percentage of the total amount withdrew (I think it was 3%). You're bank should be able to tell you what fees they charge. I found this information right on our banks website but also confirmed it when we informed them of our travel plans.
Next time we may bring less cash with us and rely on ATM's a bit more. I wouldn't worry about it too much. Just make sure you notify your bank and know your fee's. Bring enough cash to get you threw a couple of days just in case and use the cambio's to exchange any of your money and you'll be fine.
That's my 2 cents.