Thanks to the 'hounds that sent me to Taqueria El Mariachi, a fabulous little storefront of about 20 seats, where I appear to be the only Anglo customer on any given visit. Tacos especiales al pastor (pork and green chillis) come in two layers of homemade-tasting soft corn tortillas with little else but fresh cilantro and sliced avocado: a little greasy, full of flavor, muy auténtico. $6 for four tacos, almost too much for lunch (but I couldn't help eating it all).
Second visit: tacos especiales de cabeza (beef cheek), a whiteboard special, which someone obviously lovingly slow-cooked to rich, tender, fine little shreds with a flavor reminiscent of short ribs. This has a little diced raw onion as well as the cilantro and avocado. Wow!
Excellent fresh salsa, good selection of Mexican sodas (including Mexican Pepsi and Coke: couldn't tell if they were cane or corn sugar sweetened). Lovely, friendly waitstaff, Telemundo on the box (Central American soaps and talk shows make for surprisingly compelling TV!)
Taqueria Mexico is about as described: a large, dingy, dim, worn-looking Mexican greasy spoon with a few patio tables out front. I thought the Mexican here was decent and reasonable, but the place has some cleanliness issues. It is still a sight better than any bogus chain Mexican (Qdoba et. al.), but it's hard to imagine going back very often now that I've been to El Mariachi.
Great tip, 'hounds, thanks! More to come on Waltham lunch spots.