The last night of our stay in Cancun we went to the Hacienda Sisal, a restaurant about 1km from the JW Marriott, which wasn't listed in the guidebooks we had. Overall the food was excellent, and reasonably priced. We particularly liked the flautas and the Pollo Tampico. One thing to be careful of - the "special margarita " was 150 pesos, and didn't really taste that great.
Just returned from the JW Marriott in Cancun. Try El Shrimp Bucket (just across the street) for a really casual, beer and shrimp night. The Guacamole with Shrimp was awesome! Also try la dolce vita (again across the street) for a nice Italian meal; lobster ravioli was very good.