Having lived and debated both north and south of the Windy City, I have heard Chicago brought up many times as an example of how to carry out some particular aspect of urban design and planning. And without fail, somebody will pop up with the stock local pride response: "St. Louis/Milwaukee isn't Chicago! And we shouldn't try to be!"
It drives me crazy, because it completely misses the point.
Chicago, while plagued with the same problems that most Rust Belt cities must deal with, is a highly successful urban environment when compared to most American cities. It packs considerable population density across a large area, enough to support a busy and thriving mass transit system. The rail portion of that system succeeds because it runs so frequently that it's a viable alternative to driving. It's desirable not to drive because things are so dense, parking so scarce. Things are dense because a large volume of historic buildings have been preserved, and where they have been destroyed they've been replaced by buildings of greater density. It has both major chains and small local stores. It has a diversified economy, fueled by this variety of scales. It has busy street life. It has, in short, all the hallmarks of a true urban environment.
But those are not the trappings of some kind of mythical aura of Chicagoness that if emulated will turn all your citizens into zombie Bears fans. They are the basic requirements of any successful urban environment, regardless of its size. They apply across the board -- in Chicago, in New York, in San Francisco, and yes, in St. Louis and Milwaukee.
Build upon those things, and your city succeeds. Diminish them, work against them, allow developers to build sprawl instead of density, and your city fails.
It is not about becoming Chicago; it is about becoming an urban environment, and Chicago is the closest example one that works. Your city may never "be like Chicago" in whatever other ways you're thinking of (certainly it's unlikely to match Chicago's population), but it would do well to learn some of Chicago's lessons. They transcend any one location.