Plus you don't have the worry of losing a wallet full of cash. Bank machines are easy to find in Cancun, Cozumel and Playa del Carmen, so get money when you can. Every tourist town has money exchange offices that will trade travelers checks and cash for pesos. Exchange houses and banks will generally only change currency from the USA, Canada and most European currency. Citizens of most other countries should carry cash in dollars to avoid problems exchanging money.
Travelers checks are a safer option, but usually are exchanged at a worse rate than cash.
To avoid getting ripped off, shop around for the best rate and the lowest exchange fee first. You can exchange money on the street, but it is not recommended.