Well, just got back from Cozumel for probably the 30th time, not to mention other places like Playa Del Carmen, Cabo, Ixtapa, Mazatlan, etc. Guess I am just lucky since I have never had a problem
Now, I would never go trouncing around inner Mexico/border towns or go buy drugs from the locals(even in the resort cities) but that is just me.
I know some resort areas have high crime rates(Acapulco comes to mind) but if we don't go out looking for trouble, then all will/should be fine....in the places I listed. As for Cozumel, me or the wife can walk through town, around the island, wherever in the middle of the night and nothing would happen, here is even a link:
http://www.tripadvisor.com/Travel-g150809-s206/Cozumel:Mexico:Health.And.Safety.html
Not saying all resort destinations in Mexico are safe, but there are a few great places to visit and should not be discounted due to the crime rates of the rest of Mexico.