Blair S.
Yelp
I love markets, and I especially love Mercados in Mexico City. And this is the mother of all mercados. If you don't like crowds and narrow corridors, or prefer sterile restaurants, you will not like la Merced. If you want to experience the pulse of Mexico City, try some of the best soups, tacos, and snacks, stock up on cheap produce, sample moles and spices, then La Merced is a must visit. Seriously, some of the best fruits and tacos I've ever tasted came from La Merced vendors. And because of the size, prices are often the best around.
La Merced is organized chaos because there is a method to the madness, though outside it may not appear so. In the large buildings, similar merchants are generally together - food vendors along one wall, shoes and clothing together, etc. There are large sections for flowers and party supplies, livestock (Live goat? Check. Puppies? Check. Piglets? Check. Iguana? Check.), religious stuff, dry produce (beans, spices, moles), snacks, meats -- any part of any edible animal really -- household supplies... I could go on all day.
If you are at all curious about local culture, go here. It's worth the crowds (heaviest on Saturday and Sunday). Take the subway, but not on a weekend or at rush hour unless you're comfortable in tight crowds. Don't take a taxi - you'll be stuck in slow moving traffic mixed with pedestrians browsing stalls the radiate outward from the official market.
Now excuse me while I daydream about my next visit to La Merced.