Lunchtime today (Sunday), spying a full parking lot, we stopped into Azteca Groceria.
Barbacoa tacos were very good, Lengua (tongue) were excellent. You can't beat the price: $1.35 each. tameles were dismal.
A chef friend at a local caterer was not surprised--his largely Mexican-American crew prefers Azteca Groceria above the half dozen or so other taco options that line this stretch of road north of downtown Roswell and south of Holcomb Bridge.
btw: No language barrier, English or Spanish readily spoken.
A long communal table at which you can sit down and eat, or you can get them to go.
Also on the menu, but did not try: Pozzole, menudo, chicken with mole, tortas.
Other taco varieties (did not try) include fajitas and carnitas.
1230 Alpharetta St
About 1/4 mi south of Holcomb Bridge, on the west side of the road.