Hi
I live in Chicago and would absolutely endorse Frontera, the hype is valid. some of the other suggestions are also good but places like Gibsons, Rosebuds and Portillos while good food are a dime a dozen in Chicago.
If you want a great casual dining experince with unbelieveable food the best place at the moment is the Girl and the goat, its owned by Stephanie Izard who won a season of top chef, right next door is the Haymarket brewery which has a fantastic selection of hop style beer, i personally hate hoppy beer but i know lots of people who have eaten at places like Alinea, spiaggio, etc and they all rated girl and goat as their favourite. Now i will state that it is a not a tasting menu type of place you can expect to drop $80 a head for dinner. You will not get a reservation before March, the trick is to show up at 4.00 when they open and sit at the bar, if you do go try the ribs, small portion but i guarantee you will end up sucking them like a lolipop.
If you want any more info or places to check out please PM me.
Alan