I haven't seen any posts about this place before, so I'm feeling like it's a bit of a find. It's a total hole in the wall, at least from the outside -- once inside it's bright and clean. Very small, about 4 tables, a tv tuned to Telemundo for the futbol, and a little counter.
I've only been twice so far, but so far I've been wowed. The carnitas is like some sort of amazing Super-Pork, probably because they *clearly* do not stint on the lard. It's falling-apart tender, juicy, salty, just a bit crispy in parts, with a few small chunks of super-soft fat. I had their "tacos especiales" with this my first visit -- 4 small soft corn tortillas, with onion and cilantro and avocado slices and a really excellent thin hot sauce on the side. Cilantro-lovers might want to ask for a little more, but I was happy.
2nd visit I had the carnitas on a torta -- refried beans, onion, tomato, pickled jalapenos and a bit of mayo. The roll was a bit on the doughy side, not quite as good as at Tacos Lupita, but still pleasing. I also tried a regular taco with the cabeza -- beef cheeks. Unbelievable! Very similar to short ribs, with dark, deeply beefy, super-tender meat and some generous chunks of luscious fat. That, along with some nicely sweet tamarindo and horchata to drink, is all I've tried so far, but I intend to go back a LOT.
Oh, if you eat there you get excellent freshly fried chips with a very good fresh tomato-chile salsa, nicely presented in a little stone mortar. Also, the tortillas (for the tacos) were the best I've had -- very fresh-tasting and flavorful, not at all dry; I'm not sure if they make them there, but they do have a sign up saying that they sell them. But they look a bit too perfectly-shaped to be homemade. In any case, they're excellent.
The staff is very nice, primarily spanish-speaking, but pointing at the menu and using my incredibly poor spanish seemed to work fine for me. I've been the only Anglo in the place both times, I think this place has been WAY under the radar. I've been to nearby Taqueria Mexico in Waltham several times, but I think this place has the edge, at least rated on the all-important pork factor. They have a few weekend specials, including menudo and caldo de res.
Taqueria El Mariachi. 196 Willow St., Waltham. This is half a block off River Street, opposite the Gulf station, about 2 minutes drive from Russo's. Open until 10 most days, closed tuesdays.