It is very popular to say AT&T has bad signal coverage, but then again, as far as I understand the physics of it, electromagnetic radiation is not affected by the level of like or dislike for it.
If you discount those who have only ever had AT&T wireless service, those who have never had AT&T wireless service, and those who are in metropolitan or urban areas, where signal strength is very rarely a problem, you are not left with much input.
Maybe it's just me, but I have never seen a single comment where someone said, "I had AT&T service and when I switched to Verizon..." (or vice versa).