The issue with AT&T, particularly in large metropolitan areas, was a combination of multiple factors including saturation of available spectrum, insufficient backhaul to transmit data rapidly at peak saturation, and the need for more towers in general.
People in the worst market for AT&T (NYC) after the iPhone explosion had bad coverage regardless of the phone. It was difficult to see this on the data aspect of the phone because AT&T didn't commit to Android rapidly and thus had very little actual smartphone traffic beyond barely web capable Blackberries and the iPhone for a long time (until newer BlackBerry devices came out and Android became available on the network). But on the voice side of things, dropped calls became more prevalent and anecdotal reports came in from everywhere (here's one: my brother maintains two phones because he can't rely on his iPhone on AT&T in NYC for calls). AT&T even developed apps to help them identify problem areas (Mark the Spot).
The T-Mobile acquisition brings improved backhaul, more towers, and spectrum for 4G use (they aren't gaining any 3G spectrum with this deal, what they have obtained will be used for 4G).
I'm pretty sure it was AT&T and I haven't heard of similar problems with the Verizon iPhone, though Verizon's 3G speeds are slower if you have HSDPA in your area with AT&T and you get voice+data simultaneously with AT&T. It's a trade off. I was recently in NYC with my AT&T iPhone and noticed significantly greater network sluggishness while trying to access data anywhere in the city, but I'd still prefer to be able to have my phone be able to take a call and simultaneously receive network data in its cradle so it can continue to properly route me while driving.
I think your latest post nails it though (where's the buzz?), this issue doesn't seem to have been as big as the news suggested as Verizon iPhone purchases, while large initially, still constitute a very small population of iPhone 4s. The plans to jump ship were clearly not as grandiose as the Internet commenting population would like you to think they were.