We have been going to the Grand Mayan south of Cancun for 10 years and love it. It's a remarkable resort.
However, on our last trip we were the victims of the classic Mexican cop scam. While driving our rental car home from Playa Del Carmen at 9:00, we were stopped by one of the local police. He very helpfully explained how the speed limit in the left lane is 100kmh, but the right lane is only 70 kmh (which is total horse sh*t). He said there would be no ticket, but he would take my drivers license to the police station. I would have to go there to get it back, and pay a $50 fine.
Of course, we could avoid the trip and the hassle and just pay him the fine. I am well aware of how this scam works, having read other peoples' posts about it. You definitely don't want to argue with a cop in another country, and we didn't want to hassle with going back to Playa Del Carmen, so we paid up and were sent on our way with a smile and a handshake.
We really like having a car because we like to travel around the area. There's a lot to do. However, there are cabs everywhere, many shuttle services available and ... for the adventurous ... the collectivos or public buses that are everywhere.
For our trip next year, we are considering skipping the rental car and using public transportation.