Cancun airport car rental can be very tricky, as we found out.  We rented from the bottom priced place (a national brand, but not one of the top brands..). In any case, we made it to the agency’s office (not air conditioned), a one room affair, and had to wait an hour (with kids) for a car to show up… it was not washed properly, so the rental folks had to wash it (using buckets), and the car was in poor condition.  So do your home work; if herz and avis are too expensive as car rental companies, make sure you read plenty of reviews about the CANCUN branch of those companies… this will save you.

How To Avoid Car Rental Nightmares On A Mexico Vacation says It is advisable to rent from an international car rental agency instead of relying on a local agency (I 2nd that).  There are also other great hints on that blog.