La Mestiza is really good.
Coming from Southern California, I didn't expect to find any authentic Mexican food in Madison, but we gave this place a try and damn it was good. They even had nopales! Yum!
As for the people who say it's expensive: where do you eat normally? Taco Bell?
The entrees average around $13-15 and while the location doesn't exactly prepare you for that price, it is by no means expensive. My basic thought is that if you think a $6 appetizer and a $15 entree are too pricey then maybe you should look at eating at microwaved burritos at Pedro's or something.
Also: DELICIOUS CHIPS!