Fantastic for "tourism shopping" -- gigantic Mexican spice selection, generous panaderia, authentic cheeses, chili peppers with actual pep, fresh tortillas, hot food, more mangos, every Bimbo product imaginable, and soft drinks with real sugar. Plus you can buy tamarind drink or horchata to ease your nerves as you trundle through the store AND right above the Kool-Aid display, you can buy little packets to make your own tamarind drink at home.
For a full weekly shop, it's necessary to really, really know what you're doing. Packaged food of any sort is generally more expensive than at other stores, so start with the assumption that -- except for things on sale, Mexican candy, Bimbo products, and maybe cinnamon waffles -- you will be cooking from the scratchiest of scratch, as in soaking your own beans. The bean selection is awesome.
You'll need to be confident, skilled, and flexible at choosing produce, as the quality and pricing varies widely. I've gotten great deals on fruit... and I've had limes with no juice and spinach that went bad almost instantly. I am not a produce ninja.
You'll need to be a true meat ninja, and it's better if you're confident in going to the butcher's counter rather than buying the pre-wrapped meat. I have no qualms about the chicken breasts I got there (and the price was excellent) but the bone-in pork loin was a disaster, even though I've made pulled pork successfully many times. On the positive side, if you're looking for pork tongue or cow's head meat or other livestock parts that rarely show up at a big Fry's -- and then at premium costs -- you will make out like a bandit here and be the envy of all the hipsters. Also, when the seafood counter does the seafood mix that's whatever bits and bobs they have hanging around as excess, it is quite excellent and inexpensive, and I've never had a problem with freshness (with fish, if it smells, just don't buy it).
If you want to concentrate on shopping rather than be entertained by the bustle, go early on Sunday morning. Some of the side businesses like jewelry repair won't be open, but the aisles will not be a demolition derby. If you go during peak hours for the full experience, definitely start by getting your agua fresca to ward off dehydration, as you'll be here a while.