One of the best places for authentic Mexican cuisine. Its tucked in the back of a small grocery store. Just walk to the back and seat yourself. If you want something other than water you grab it out of the coolers to the right before you walk in to the restaurant part.
The have really good tacos. I like the steak ones with onion, cilantro, and lime. Both the green and red salsa they serve with the tacos are really nice. Their chicken tostadas are also tasty. The chile rellenos are really good too; lots of yummy sauce and plenty of rice and beans to go with it plus a side or corn tortillas.
We've been to other small, authentic Mexican restaurants around Madison and this one is right up to par with those if not slightly better and super affordable. A nice location in the westside; close to our house and work so that's one of the reasons we choose it often as well. Its nice not to drive far for authentic Mexican fare.