When I was preparing for a trip to Mexico, I was very concerned about renting a car in Cancun. I have rented cars in other parts of Mexico multiple times without a problem, but the comments listed on TripAdvisor and other places suggested that nearly all the rental companies in Cancun were terrible and unethical. Here is a detailed description of my experience.
I decided to book with Avis. I didn't want to worry about insurance so I used Amex Premium Car Rental Protection which was only $25 for the entire trip. I also joined Avis Preferred because I could set a travel profile and I figured the agents would be less likely to mess with a frequent renter. In the profile I included Additional Liability Insurance for peace of mind and declined everything else (in keeping with the Amex directions).
When we arrived - yes the Cancun airport is an experience. It was clean, it seemed small, but there is a lot of activity - the rental car companies are calling out for your attention. When I stepped up to Avis and mentioned I had a reservation, I was quickly ushered through the crawd of timeshare sellers and taken to a special spot where I was soon picked up. Very friendly driver, free water was handed out. I tipped him as I would in the US.
The Avis lot is about 3 minutes away. All the agents were busy (maybe 5 of them). I went to the preferred line and saw that the agent handled the preferred line was busy dealing with some issue. I waiting about 2 or 3 minutes and then I noticed that the van driver had come in and got the attention of the agent to make sure I was taken care of. I was handed off to another person. I handed him my Amex card and my driver's license. He started to ask about insurance and I simply told him I wanted coverage that were in my profile. His manager leaned over and told him to move along. Everything else was relatively quick. They were trying some system were the rentals were processed with their cell phones. This was a bit slower than I think they planned, but I was taken to the car, which was of the quality I would have expected in the US. Clean, filled with gas. I looked at the receipt and there were no surprises.
It was a very good experience, particularly in the light of the low expectations I had. The return process was equally good. I had read (and heard from a B&B) owner that I should NOT believe the car rental companies were open 24 hours despite what they say or put on their web sites. We had checked and rechecked multiple times...always the same answer. We showed up at 4am and there was a staff of 3 or 4 at the place. Check in was easy (if maybe just a bit slower than in the US) we were in the van in under 10 minutes. The agent seemed to go out of his way to show me exactly what I was paying (in the low $200 range for a week).