This has to be somewhat location specific. AT&T seems to have some of the best reception where I usually spend most of my time, the Gulf and East Coasts. The iPhone seems to get as good or better reception than my old Nokia (disclaimer: this was a very old Nokia). Apparently it is terrible on the West Coast.

Of course these are isolated data points. I do think that Apple has made a mistake by limiting the iPhone to one carrier, especially one that can arouse strong hatred like AT&T.