I rented a car at CUN airport and drove down to Xel-Ha and return. Safe roads and most drivers were normal. Oh yes, there are a few agressive ones that will pass with extreme disregard for safety but I see that in many European countries as well.

The more appropriate word of caution is to be sure of the safety of your rental vehicle. Even from big names such as Avis and Hertz, be sure to check out tire wear, wiper useage and brakes before driving off the lot. Cars generally have much more wear on them in Latin America than other places in the world.