The opening is, in my view, among the strongest from any Chicago song. The instrumentation throughout is fantastic, really a microcosm of the Chicago feel. You feel like you're transported to the streets of Chicago in an impromptu street party on a bright, sunny morning, being greeted by complete strangers, and, had Starbucks existed in 1978, maybe the barista would greet you outside the door of the shop with a free latte.
This song, which was later used by CBS Sports for its NBA coverage, has to be among the list of best "feel-good" songs ever. I dare you not to feel better after playing it once. You might find yourself playing it again. And again. Pun kinda intended. :)