If you zoom in on Phoenix, AZ from Clear's coverage map, there are 6 towers in the Metro area. 3 in the southeast (Tempe, Mesa, Chandler). 2 in the north (North Phoenix and North Scottsdale, and one on the westside on interstate 10.
I spend a lot of time around the North Phoenix Tower (N. 7th St and E. Bell Rd.) and get a good 4G signal on my Nexus S 4g.
I have not checked the others.
BR