Congestion should only mean it drops calls and loads data slowly, I would think.
A more fundamental network problem will show up as dropping out signal and low number of bars.
I think the real problem with AT&T is most of their 3G network is on 1900mhz, which is a very poor spectrum.
I live in Australia, and we have the iPhone on all the major carriers. Telstra's 3G is entirely 850mhz and you can really tell the difference to the other 2100mhz carriers. It works far better in skyscrapers and car parks. And even works well in the middle of no where in the outback, iPhone is nearly always 5 bars 3G on Telstra. Who have the world's best 3G network with 99% coverage of the population of the country and speeds up to 21mbit currently being upgraded to 42mbit, and this is important because in outback 3G is many peoples only broadband option.
So I can say with a lot of confidence, the problem in the US is AT&T.