Reyna's is not just a Mexican grocery -- it also carries a lot of products essential to making Caribbean, Latin American, and other such foods. But in its heart and soul, Reyna's is primarily an excellent Mexican grocery store. The fresh tortillas are excellent, the prices on the Mexican cheeses are literally a third of what inferior Mexican cheese costs at the supermarket, and the sauces and salsas are almost uniformly delicious. Plus, if you're hungry, you can usually get a pretty decent snack here. They have tamales/burritos/tacos/etc., and also dessert-type stuff like conchas.
This place is completely indispensable if you're at all interested in cooking even remotely-authentic-tasting Mexican dishes and you live in Pittsburgh.