We had a 7 hour layover - originally intended to take the subway in and didn't realize that the subway station is in Terminal 1, not Terminal 2 where the international flights are. You can take the monorail to Terminal 1, and the subway from there, but the Metrobus is so fast and convenient I'd only do that if I was literally counting pennies (Metrobus fare is 30 pesos, subway is 5). The card itself is 10 pesos and can be used for multiple people, and machines are available inside the airport as well as by the stop.
There's room for luggage on the bus, and a security guard on every bus and some stops as well. The bus never filled completely.
I was concerned that we'd get stuck in rush hour traffic coming back, so I timed the trips - at 2 PM, it took us 35 minutes to the Republica de Venezuela stop. at 5:30 PM it took 40 minutes to get back to Terminal 2, but 10 minutes of that was sitting at Terminal 1. It was incredibly fast and efficient - we passed countless taxis going each way (Metrobus has a dedicated lane and even its own turnabouts). The buses run every 15 minutes.
While it took forever to get luggage (it gets examined by drug-sniffing dogs in batches - I think we waited over an hour for our bags to come out), the longest and most painful part of the layover is up front for immigration and getting luggage - getting through customs was quick, rechecking luggage back in doesn't take too long (we were told to go to the line for airline staff to recheck in for Aeromexico, don't know if that's the case for all airlines.) Coming back in through immigration and security was fast, with no lines at 6:15 PM on a Thursday night.
There are decent looking tacos and tortas right by the Republica de Venezuela stop, so if you had a shorter layover and just wanted something to eat, you could eat there and hop back on the bus going to the airport.
We wandered around a bit and found the area around the National Palace Museo del Templo Mayor to have good street food offerings (we'd originally headed the wrong way from the Zocalo and ended up in an area that was way too upscale to have street food. We did buy some chocolates and chili mango slices from a department store though). We eventually ended up in an unmarked alley (next to Merceria Hermi at Venustiano Carranza 121) with multiple food vendors where we had good taquitos and gordita, and an excellent cheese quesadilla. We forgot all the recommendations not to eat shredded lettuce, scallions, etc but neither of us had problems later.
We then were drawn to the rotating spit topped with a pineapple outside Taqueria Del Centro on Correo Mayor (16-G) - my husband always complains that you can never find a proper al pastor where we live (rather than getting properly singed by flame and shaved, we usually end up with thicker slices finished on a griddle). There's seating inside. Tacos are 10 pesos each and very, very good - the meat is sliced paper thin, with a super thin slice of pineapple. Tortillas were very thin as well - I don't know if this is a regional difference or not. I tend to prefer thicker corn tortillas, but since the meat was shaved so finely the thin tortillas worked. We returned later for a torta, also excellent (I find tortas at home to be too much - too much bread, meat too thick, etc - proportions were more to my taste here).
We spent too much time eating and never made it to Templo Mayor, so we consoled ourselves with another taco from a stand nearby (campechano, also good, also with thin tortillas).