I've eaten here more times than I can count, and I can easily say that it is some of the best Mexican food I've had outside of Texas, California or the Southwest. There is a reason they keep winning the ""Best Mexican Restaurant"" category in the RFT.
First off, they are not serving the usual tasteless, bland type of Mexican food that you find at many of the Mexican restaurants in the Midwest. Their food is correctly spiced and well prepared.
If you only like ground beef tacos and such, stick with Taco Bell. But, if you want want to try the real thing, order their Tacos de Carne Asada which are made with chopped steak served inside three soft corn tortillas with all the condiments on the side so you to prepare them the way you want.
I can also highly recommend their Chicken Soup which is not like anything your mother made unless she was from Mexico. Also, any dish on the menu which includes their spicy chorizo is worth trying. My favorite is the Chori Pollo, a grilled chicken breast topped with chorizo and melted cheese.
My only complaints would be that some of their red sauces have cinnamon in them which I don't care for, and their margaritas could be better, especially considering the high quality of the food. I wish they would start serving the Topolos margaritas from Frontera Grill in Chicago (recipe is on their website: hint ;)
Last comment, this is the cleanest Mexican restaurant I have ever eaten at anywhere in the USA, and I travel a lot.