The coverage definitely varies, even within the same general area. Where I live, the coverage is good [six years ago, you had to go outside to use a(n AT&T) cell phone, but that was six years ago]. Where my university is, there are huge dead zones, especially inside some buildings... Of course, the biggest dead zone I know of is anywhere not by a window in the library, and I'm sure they don't want people making calls in the library anyway. I also haven't compared it with my friends' Verizon to see if it's something about the architecture itself of some buildings that blocks the signals or if it's just AT&T.
On the other hand, the coastal states are AT&T's strongest zones on the maps (my whole state is supposed to have 3G coverage), and I have received some text messages hours late on my campus. It is frustrating, especially since often it takes receiving another text message to "push" the stuck message(s) through. Also consider the fact that most college campuses now use text messages to communicate news of emergencies, and dead zones suddenly become dangerous.
However, I'm sure there are areas where Verizon sucks, areas where T-Mobile sucks, etc. It also could just be the awful phone I have. It just happens to be I "found" a big area with spotty coverage...
And, iirc, it has to do with the architecture. Verizon's 3G network has a longer range, but slower speeds (it makes sense that that would be the trade-off), and the television tells me that you can't talk and data at the same time (can a call interrupt you while you're using the data?), so it kind of seems like there are trade-offs to be had.
