It's important to differentiate, when writing about beer, between high and low-brow beers. When you finish reading this post, please don't think I was trying to argue that Old Style falls in the former category. It is a low-brow, 83-cent-per-pint-can-at-Jewel-Osco beer.
It's also important to differentiate between different local brews. Chicago has its share of respectable microbrews, from 5 Rabbit to Metropolitan's Iron Works, but arguably the most distinctly "Chicago" beer (ignoring that it's brewed by Pabst in Wisconsin) is Old Style. The beer has been the unofficial beer of Wrigley Field and the Chicago Cubs since 1950 and it's on draft at most major (or friendly neighborhood) bars in the city.
It's comparable in look and taste to a MGD, and at roughly 4.7% ABV it'll get you drunk as good as a MGD, but there's something inexplicably Chicago and inexplicably right about kicking back with an Old Style and watching the Blackhawks or the Cubs (the former now sadly eliminated from the playoffs, the latter…well…you know). Their slogan is "think local, drink local" — pair it with a burger or a Maxwell Street Polish and you're all set.
Should you stay or should you go?: If you're still unconvinced, just watch this "old style" (see what i did there?) commercial.