Never had a problem with the ATM...bank card or credit card...always have used the one at the small convenience store....just use the same precautions you would in any big city...no 3 am withdrawls...you never know whose watching...
In party central there are atms that use english. Not sure exactly where they are, but they are definitely out there.
Oh and remeber that your balance will be in Pesos too! So no, some rich uncle didn't leave you enough to stay forever in Cancun. Jamie