I'd ignore most of these comments and read the U.S. State Department report. I don't normally say that, but as someone who lives in Mexico now and travels around the country regularly, there's no other document out there that is as granular and detailed as that one (without paying lots of money for a private one). They've done an admirable job of not only telling you state by state where the troubles are (Yucatan and Baja Sur, no worries, Michoacan and Chihuaha a different story), but also which areas within a state should be avoided if you have a choice. In broad terms, every resort area is trouble-free except for Acapulco. In all of them, less crime than a comparable place in the USA like Miami or Myrtle Beach. Almost anywhere a U.S. tourist would go is fine. The number of foreigners affected by violence in the country is extremely low.

Mexico City has far fewer homicides these days than Washington DC and JDiver's comments above are about 10 years out of date. Unless you're a billionaire Mexican, the risk of an express kidnapping is close to zero and unless it's late at night, I always hail a taxi on the street. I've been doing it for years, as has every expat I know who lives there.