Your own bank will probably charge a fee for using ATMs abroad; the foreign bank you use may also charge a fee. Nevertheless, you'll usually get a better rate of exchange at an ATM than you will at a currency-exchange office or even when changing money in a bank. Extracting funds as you need them is a safer option than carrying around a large amount of cash.
PIN numbers with more than four digits are not recognized at ATMs in many countries. If yours has five digits or more, remember to change it before you leave.
ATMs (cajas automáticas) are commonplace in Los Cabos and La Paz; Loreto and Mulege also have an ATM. If you're going to a less-developed area, though, go equipped with cash. Cirrus and Plus cards are the most commonly accepted. The ATMs at Banamex, one of the oldest nationwide banks, tend to be the most reliable. Bancomer is another bank with many ATM locations.