My apologies if this posts twice- I'm stuck on IE6 at work.
What pito said- each wireless carrier works well in some places and poorly in others.
I worked in a Verizon store last Christmas season to pick up some extra cash. I lost count of the number of times I heard "The Droid is great, but I'd rather have the iPhone with you guys" or "I heard you're getting the iPhone." I can only speak for where I live, and here, there'd be a huge exodus to Verizon. The Verizon staff know it, the AT&T staff know it- everyone knows.
Doesn't mean it'd happen everywhere. A lot of my family live in another city and most of them are on AT&T and seem to be happy with it (though I don't think any of them have an iPhone, FWIW).
Hey Stephenson, guess how Sprint got themselves in the catch-up position they are in right now, struggling to regain their customer base? I worked at a Sprint call center a long time ago and half the managers at the time basically said "don't take too much time saving one customer- there are plenty out there."
The customer you can afford to lose is rare in the extreme- even a PITA that costs you a lot of money to keep might be worth it if he raves about how much you're willing to do for him to keep his business. Good word of mouth is the best advertising, and saying ANY customer is not important is a really, really stupid move (even if it is true sometimes).