In terms of 4G, I get a single bar indoors ("fair") in the Printer's Row/South Loop area, and that's on the tenth floor with big floor-to-ceiling windows, but many other buildings around. But at home, it's just easier to use wi-fi. Outside, at street level, I can get a stronger signal, and interestingly enough I was inside Lincoln Hall at a show on Saturday night and got a perfect 4G signal! Still, even with one 4G "bar" at home, the speeds are good.
3G coverage seems fine so far downtown and in the South Loop. The phone will sometimes show no bars but I think it just powers down the radio sometimes. Not had dropped calls or too much breaking-up/crackling. Overall, significantly better than the 2G iPhone with AT&T I came from.
Not really had the opportunity yet to check it inside buildings in the Loop where AT&T really suffered... doubt there'll be much difference. It's so odd: your buddy with his Verizon phone can get a signal in one spot where you get nothing, then you go up the street and the opposite happens.