If you want some authentic Mexican, this is THE place to go in Madison. I was incredibly impressed by the lengua and chorizo tacos, and the "mi mexico" platter is a great value for $10, giving you a BIG portion of flank steak, grilled nopales, beans, and peppers! I didn't have a chance to try the tortas or huaraches, but watching them cook up on the flattop grill was awesome.
Two warnings:
1) This is a converted row house, so there isn't that much seating. There's a couple of small tables and a big countertop / bar that runs around the grill.
2) When they're really going and have all of the gas stoves and grills going, you get a very distinct whiff of gas in the place. It didn't bother me too much, but some people may not like that.