Heh, I have mixed feelings as some one who has no plans to move (I am already under contract and I have no qualms with AT&T so no motivation to pay the money to change. Even when my contract is up, it would depend on what Verizon offered vs. AT&T. AT&T might win out if they have the more updated phone at the time long as they don't mess up my current plan cause easier to not change carriers and I'd rather have the more updated phone plus I like having phone+data at the same time, unless Verizon offers something really tempting over AT&T. And honestly, by the time my contract is up, Verizon might have better technology that solves the voice and data at the same time problem).
The bad:
Maybe some of my friends/family with AT&T might move which will make me use more minutes cause most of the ones I call the most are on AT&T. Not a huge deal, I don't call much anyways.
The good:
Even discounting the idea that AT&T's network might get better, this means the iphone gets more users which is always good for all iphone users cause more motivation for developers to make stuff for the iphone. So that's a huge plus right there whether you prefer AT&T or Verizon.
Competition for AT&T. This will help keep them motivated to offer decent plans hopefully as they no longer have a monopoly on the phone. Good for us AT&T users

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Oh, and three of the people I call the most have iphones so if I really was crunched for minutes and didn't want to pay the higher minute plan, just try face timing them more

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So overall, even if you like AT&T and plan to stay, this is good for everyone.