Once is a very busy part of a very busy city. Its tiny sidewalks are crowded with people hurrying from A to B. Hyper specialized shops sell seemingly random items in bulk from balloons to beads.
An entire block of shops is dedicated to bolts of fabrics, another to party supplies, another to rubber animal figurines and toys.
There are window displays filled with mannequins in sequins from top to bottom for both weddings and quinceañeras. This is an area to wander and people watch.
You can shop if you'd like but check for minimum spend signs on storefronts, usually scribbled with a sharpie "minimo 30,000 pesos" or something along those lines. I come here to shop for my toddler birthday parties. Others come for upholstery fabric for home redesigns. You name it, Once has it.
Comef or a wander, people watch, it's a unique neighborhood.
Safety: This is a safe neighborhood but like in any crowded city situation be aware of your belongings. Keep your bag close and zipped tight.