Mexico city is a place where as a film photographer you can find anything almost anywhere: lots of films, urban landscapes, rural landscapes, historic places , secure locations, and people that doesn't care what you are doing in the street..
There are a lot of downsides using film as your main medium when traveling, from insecurity to film scarcity, how easily you overcome this issues depends a lot on the resources the city can give you.
Mexico city as an average big city is a place where as a film photographer you can find anything almost anywhere: lots of films, urban landscapes, rural landscapes (inside the city), historic places (modern, colonial and pre-hispanic), secure locations, and people who don't care what you are doing in the street, this makes it a very film friendly city...
Portrait Photographers will find a lot of sui generis people around that can be asked for a snap without getting a no for answer, there are places like "La ciudadela" or "Plaza de la constitución" where people dance traditional dances and play pre-hispanic music and they are ok with posing for photos, as for common people it will require more effort to convince them to pose but if you already have experience in speaking your way to it then the probabilities are high.
Street Photographers will find that any place is a good spot to snap some urban activity, but there are some places that are better to avoid like "La Merced", "Tepito" or "La colonia Doctores" where unfortunately the best photos can be taken.
There are a lot of historical places to take pictures without any permission or problem, some museums don't allow the use of Flash and others require to register your camera, some good places i like to go to are "Plaza de la constitución", "Coyoacan", "Chimalistac", "Roma Norte" and "Palacio de Bellas Artes".
In general if you are visiting Mexico City don't hesitate to take your camera with you everywhere, there are astounding locations for portrait photography, cool historical sites and lots of urban activity...