Very simple and delicious. In our tours for the last eleven years, we haven´t received any complain because of diseases. Everything is well done and cooked. Usually grilled steak (carne asada), chiken with mole, different kinds of tacos and quesadillas, cecina (a kind of soft dry steak) and lots of the most tasty tortillas of blue corn. You´ll find different brands of bottled water everywhere, and if you try to avoid certain fresh vegetables, youll be safe.
In the other hand we have noticed a very bad habit in foreigners, that end up sick accusing to inocents. We have seen foreigners that are served with canned sodas or beers, and they don´t clean them at least with a napkin before drinking out of them. They just open the can and raise it to the mouth, just like that, dirty. And then they complain about the lettuce or the Moctezuma´s revenge.