The answer to this question varies depending where you visit. Cancun, Cabo, and other heavily touristed areas may use USD as well as peso.

This poster is going to PV, where peso is what he/she'll want to use.

OK, lest you have not yet received the message:
We are leaving our house at 03:30 on Monday: BJX-DFW-JFK-MAD-BCN.
Then, after 2 nights in Barcelona, we are doing a 10 nt. Med Cruise; 3 nights in Taormina; 4 nights in Ravello; 3 nights in NYC before returning home.
We have MXN $1100.pesos; US $16.; 3 Euro in cash leaving our house. Plan to get USD and Euro via free USAA ATM and use our CC (Citi in the US for AA miles; USAA in Europe).
Buen viaje.
M