I agree w/SAT Lawyer. If you are in the hotel zone, use the bus, or a cab if you want to splurge. You can get to restaurants, downtown Cancun, Walmart, the ferry to Isla Mujeres, all via the bus.
I have heard so many bad stories that I will never rent a car in Mexico. If you have the misfortune to be involved in an accident, it will be your fault. Automatically. Rich man pays. And you will likely be expected to pay up on the spot.
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13F
PS This thread should be moved to the Mexico Forum. Cancun faces the Caribbean, but there is a better place for this thread.