Most restaurants and stores accept Visa and Mastercard. American Express is also accepted. However, be sure to bring cash with you for those merchants who don’t take credit cards.
Also, be sure to call your credit card issuer prior to departing for Mexico, just to let them know (it’s a total hassle to arrive and find that your cards have been placed on hold due to ever-stringent security at your issuer). We usually take the time in-between flights to make the calls.
Note that many credit card companies charge a 2% conversion fee for foreign exchange transactions. There are several that do not, Capital One being one of them.
When buying things in Cabo, insist on being quoted in PESOS. If you’re being quoted in dollars, the merchant will likely convert to pesos when tallying your bill and you can bet that the conversion won’t be in your favor.