Where I live, Verizon have reliable and comprehensive 3G coverage.
Where I live (within easy driving distance of their world headquarters, BTW), AT&T do *not* have reliable coverage at all. To the point where I cannot make or receive a call on an AT*T cellphone in my own home. 3G is nice on paper, but you don't see it around here even when you do have a signal. This is why I don't use this provider.
Case in point - Comparing my Droid to a coworker's iPhone: I can stream audio (Pandora is free and niiiice), make/receive eMails, and/or surf the web at the same time. Now, I may not be able to make a call at the same time... But neither can the iPhone, regardless of AT&T's implications stating their network can.
AT&T are butthurt because (in this case) Verizon's attack adds are right on target. The service involves more than just the device. In this case, the 'Droid's few failings (lousy keyboard) are far far far more than offset by the size/availability of 3G coverage.
/end_discussion