Eeeeeh, no. Chicago's got so much excellent Mexican food, but for the love of god don't eat in a Wrigleyville frat boy place.

If you want to get some good Mexican food, look for a local hole-in-the-wall taco place that has lots of Mexicans eating in it. Most Chicago neighborhoods have a bunch of them. And be sure to drink an horchata while you're there.

Also, you could take the Pink Line to 18th St or Damen and get amazing, cheap Mexican food down there. I'm a big fan of Los Comales on 18th just east of Ashland.

In general, the better restaurants are not in the Loop or River North (there are some good places, but very touristy and generally expensive) or Lincoln Park/Lakeview (very gentrified neighborhoods with disappointingly bland food catering to drunk ex-frat boys & sorority girls who grew up in Ohio). Chicago has many outstanding ethnic restaurants, but they're mostly in neighborhoods with lots of immigrants and relatively low real-estate costs.

Some of my favorite places, in no particular order:

El Tinajón (Guatemalan):
http://www.yelp.com/biz/el-tinajon-chicago

Los Comales (Mexican, already mentioned):
http://www.yelp.com/biz/taqueria-los-comales-3-chicago

Ann Sather (absolutely massive, ginormous, Swedish-themed breakfast place - they have one in Lakeview and one in Andersonville, both are institutions):
http://www.yelp.com/biz/ann-sather-chicago

Phoenix (Chinese, best for dim sum, but their dinner offerings are decent, too):
http://www.yelp.com/biz/phoenix-restaurant-chicago

Edit: Just saw chgoeditor sort of seconded my Lakeview/Wrigleyville opinion (albeit in much more diplomatic language.) But his/her excellent rec of Angelina's reminded me of another Italian place I quite like:

Anna Maria:
http://www.yelp.com/biz/anna-maria-pasteria-chicago