Although I've only been to Cancun twice, I've been to Mexico seven times, and have two more trips coming up in the next 8 months. The resort area of Cancun, the skinny island shaped like a seven is safe if you have your senses about you. The city, though has some tough parts that I would not go. However, the downtown area has some good restaurants, and Mercado 28 is an experience, so I think every one should venture out of their resorts. If you want a taxi in Mexico, it's best to get them at their cab stands, called Sitios. Every cab has the number of his sitio on the cab, and a cab number. Verify the fare before you get in. Most cab drivers are just regular guys trying to put food on the table, but you can get some rogues. More in Mexico City than Cancun.
I'm not a big fan of Cancun. The beaches are beautiful, but there are too many American chain restaurants and not enough locally owned in the tourist zone. I highly recommend El Fish Fritanga at KM 12. If I go back to the east coast, I may try Playa del Carmen or Isla Mujeres. Cancun is too American.
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I thought getting old would take longer.