I've heard a few stories of having to bribe the local police. I'm not sure I would want to go to a country where I can't walk around and actually experience the culture. What's the point of going to a resort you can't leave? Might as well go hang out with the mexicans at the local restaurant then.
Cancun is really touristic and pretty safe. I have been a few times. You can leave the resorts and have no fears.
It is beautiful area and there is something for everyone. Pyramids, nature, cenotes, parks, adventure parks, night clubs, scuba diving. I would highly recommend it.