PC Magazine recently evaluated the data network performance of a number of wireless carriers in cities across the US. The full story shows how widely performance varies, as illustrated below. While AT&T wins the best combination of coverage and performance, it faces clear competition from Sprint's 4G network rollouts and T-Mobile's upgrades to HSPA+.
These are the words of Robert Morton, one of Tableau's software engineers. Check out his informative visualization below.
Each of the bars on this visualization depicts the average (fat part of the bar) and maximum (smaller part of the bar) values for either the download speed or time to first byte. The average across the entire network is represented by the black reference lines. As you can see from that reference line, Sprints 4G network beats AT&T and T-Mobile in download speed, but since it is only available in limited markets it lost out to AT&T overall.