I was just about the last person on Earth to get a cell phone back in 2005, and chose a Go phone Razr from AT&T (then Cingular), so I've always had AT&T and can't make a comparison. I know plenty of people who switched for the iPhone, and I do hear them complain, but I think the benefit of having the phone outweighs dealing with AT&T's BS.
But, definitely, I would not have them if it weren't for the iPhone. When switching my account over from Go Phone to iPhone back in October '08, they royally screwed it up in ways I can't even describe, and to this day my autopay STILL doesn't work, despite the fact that I call them every month and re-enter my info and it's always said on the website that I'm enrolled and it's active, but EVERY month it fails. And I get tons of "Could not activate cellular data network" errors and have to restart my phone 1-2 times a day.
Yeah they suck. But I've never had any other wireless carrier, so I don't know.