I just got back from Riviera Maya. I used cabs to go to Playa del Carmen to dive. Two things to be aware of if you do that:
The hotel we were at was giving 13.50 pesos to the dollar. The cabs were charging $370 pesos or $37 USD, which is a 30% higher exchange rate. So, it's worth paying the cab in pesos if you can get a better than 10-1 exchange rate.
The cab drivers are eager to work. They would ask when I'll be done and can they come pick me up. I didn't try to bargain with them, but that may be an option.
There are two police checkpoints on Rt 307 between Cancun and Playa del Carmen (that's as far as I got, there may be more further south). The checkpoints are like toll booths, in that you have to stop.
They randomly search cars and trucks, although it looked like taxis and tour operator buses get in the right line and go right through. The cab driver told me they're looking for criminals which may be true. I just mention it as something else you will have to deal with if you rent a car.
Have a great vacation. Cancun airport is so much more "fun" than American airports.