I have been an avid reader of these forums now for the past 3 years (started contributing much more recently though) and have seen a zillion and a half whiny posts about how terrible AT&T is, yadda, yadda, yadda. I've just never understood it personally. Am I just the one lucky guy in the crowd? I seriously doubt it.
I travel a lot for business and for pleasure...and I would never think of leaving home without my iPhone. I bring it with me everywhere, all day, all night. I've also not experienced problems with AT&T in my travels. Boston, NY (city and state), Wash DC, beach in North Carolina, mountains of West Virginia, Minneapolis, Las Vegas, all over Florida, around Dallas Texas, San Francisco, LA, Ohio....I get connected everywhere. The worst I've seen in the past 3+ years was that the connection was lost deep in the mountains briefly, or the server stopped responding due to high volume maybe a dozen times...which usually lasted less than a few minutes.
Over nearly 4 years and across a good selection of the country? I seriously can't imagine that being deemed "bad" service, but people will say it nonetheless. I am always left with the impression that someone posting how "rotten" AT&T is makes a point of amplifying their grievance...and we aren't really getting a good picture of things just reading about complaints on a forum. Sure, there are likely areas without coverage, but really...it is in no way dire like folks try to make it out to be.
Watch someone come along and start calling me an AT&T fanboy for saying that, in my experience, I think their network has very much done me well personally. I've been satisfied enough that even if the iPhone went to Verizon (my former carrier), I'd stay with AT&T and not think twice about it.
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If you haven't bought an iPhone yet....it might actually be the safe bet to wait another 2 months and see what the next generation is like. In my estimation, no other phone out there could replace my 3gs...the competitors are just not attractive to me at all. I definitely am looking forward with anticipation as to what Apple will bring forward this spring though.