As of February 2012 the exchange rate was around 12.5 pesos per dollar. Keep in mind that if you are using dollars the exchange rate may vary from on place to another. The best ATM (used by the locals) is the one behind the Oxxo c-store. 

Best Currency - As you generally have to exchange anyways, may as well get pesos. Although most if not all places take USD, they generally give you your change in pesos. Plus the “Street Exchange Rate” (The rate of exchange at most shops, restaurants, etc) is 10 pesos = 1 USD. Today’s rate (July 27th 2009) for example is 10 Peso = 0.753666 USD (As per http://www.xe.com/pca/input.php ). So as you see you will be getting less “Bang for your buck”.

You can also use the ATMs there (At the Marina) & they give fair exchange rates & will dispense you your $$ in pesos. Best to check with your bank to see about service charges to use ATMs in Mexico however as some can be a little steep.

Bring pesos  - You will enjoy a minimum of ten percent advantage.  Every exchange from dollars to pesos costs you money. Each step, someone is making money.

Go to your bank in the U.S. a couple of weeks before your vacation and order some pesos.  The bank will get them and sell them to you.  Bring only Pesos. Okay a few U.S. dollars if you can't stand it.  You won't need it!

The Mexico Government has made it very difficult to use dollars down here. In the old days you could expect to get your best value using dollars. Those days are over.  A Mexican citizen cannot exchange more than about 1000.00 U.S. per month without having to pay a tax.

Using your credit card used to be one of the best ways to get a good exchange rate. It still is. It's advantage is offset by the "service fees" most banks charge when used outside the U.S.  That tages another 3% on to your purchase.

Book your excursions directly. Not through the hotel. If you are just buying tickets to Xcaret it probably won't matter. You are just paying for admission.

With excursions like diving, snorkeling, sailing or fishing, it is much better to do your own research here on TripAdvisor and book direct. The hotels charge exorbitant comissions from these concessions. The result is that those exorbitant comissions have to come out of somewhere. Either the prices or inflated or the quality of the boats (tackle, equpment, etc.) will suffer.

Take advantage of the excellent dollar/peso exchange rate and visit!

Cheap flights and you can find absolutely fantastic hotel deals right here on TripAdvisor!