From the U.S. State Department:
Millions of U.S. citizens safely visit Mexico each year for study, tourism, and business, including more than 150,000 who cross the border every day. The Mexican government makes a considerable effort to protect U.S. citizens and other visitors to major tourist destinations, and there is no evidence that Transnational Criminal Organizations (TCOs) have targeted U.S. visitors and residents based on their nationality. Resort areas and tourist destinations in Mexico generally do not see the levels of drug-related violence and crime reported in the border region and in areas along major trafficking routes.
...(C)rime and violence are serious problems throughout the country and can occur anywhere.
Guerrero: You should defer non-essential travel to the northwestern and southern portions of the state (the area west and south of the town of Arcelia on the border with Estado de Mexico in the north and the town of Tlapa near the border with Oaxaca), except for the cities of Acapulco, Zihuatanejo, and Ixtapa. In those cities, you should exercise caution and stay within tourist areas. You should also exercise caution and travel only during daylight hours on highway 95D (cuota/toll road) between Mexico City and Acapulco and highway 200 between Acapulco and Zihuatanejo/Ixtapa. In Acapulco, defer non-essential travel to areas further than 2 blocks inland of the Costera Miguel Aleman Boulevard, which parallels the popular beach areas. In general, the popular tourist area of Diamante, just south of the city, has been less affected by violence. Flying into the coastal cities in southern Guerrero remains the preferred method of travel. You should also exercise caution in the northern region of Guerrero (the area north of the town of Arcelia on the border with Estado de Mexico in the north and the town of Tlapa near the border with Oaxaca). ...
U.S. State Department Warning