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From my experience in AZ, where pretty much everyone was using verizon as they supposedly had better coverage, AT&T worked pretty well for me as I could keep using my regular phone and the call quality was good and I had very few dropped calls. But as I was using a prepaid sim it got way to expensive. So I switched to a cheap verizon phone and it dropped calls like crazy, especially international calls, >50% were dropped.
 
No cause I'd like to hear how their coverage is compared to Att.

The coverage would be the same as it is for a US customer.

I haven't visited the US since 2006 (it was Cingular back then) and I was only on a 2G device, but the coverage was good in the places I visited (Las Vegas, Honolulu, Maui, San Francisco, Hollywood, Anaheim, San Diego)

The main difference might be that people with a non-US SIM could roam onto T-Mobile or AT&T.

My carrier allows people to roam onto either. In a place with a poor AT&T signal, my iPhone would use T-Mobile and vice versa.

You probably wont see many people comparing data experiences, given the HUGE cost of using that abroad.
 
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