I'm in Aurora (just outside of Denver), and I have an iPhone 3G on AT&T as well as a N900 on T-Mobile.
In general, T-Mobile seems to offer higher data speeds but less coverage area vs. AT&T. And it seemed AT&T was far behind in 3G deployment a few years back, compared to their CDMA counterparts (Sprint, Verizon). All things considered, I'd give AT&T a 7/10 for network. I've only hit extreme congestion once, at Coors Field during game 4 of the World Series....not something unexpected. However, the customer service, available voice/data plans, and general attitude of AT&T force me to give an overall rating of 5/10. When it was Cingular, I remember having a better customer experience, though the network was going through growing pains.
The sad thing is, bad experiences with Sprint in this area and Verizon in Hawaii have resulted in a sort of wireless "limbo state" for me....I'm not completely satisfied with the 2 carriers I have now, but I'm not drastically inclined to drop both and give either Sprint or Verizon a mulligan. Eventually I will settle on one carrier and drop the other, likely keeping T-Mobile....better pricing, no contract, higher data speeds, good customer service, and a better handset (N900) for what I want.
Certainly do not count me in the 73% figure noted in the summary, as I neither took a survey nor agree with an averaged rating over 6/10 for AT&T.
I'm very interested to see what happens to Verizon's network when they get the iPhone.....