Cancun works on both Mexican pesos or American dollars. The are no direct debit machines in use. And you can find ATMs at MEX airport, or at major banks in Cancun for cash withdrawls: both Cirrus and Plus.

Beware of using credit cards at stores and restaurants, and even at hotels they have been known to make double impressions, or keep a skimmed duplicate of your card info. You are helpless against such scams, so just call your credit card company if they show up on your statement after you get back. They are used to such things happening. It won't be your fault since it can happen to the most vigilent of us. For that reason, just use your credit card to check into and out of your hotel, then use cash for everything else.

As for exchange rates, they are pretty fixed and the peso is a fairly stable currency these days. You will find little difference from cambio to cambio. Your best bet is to use an ATM to withdraw what you think you'll need for expenses. Travellers cheques in US dollars are a good alternative.