I've never had any trouble with AT&T coverage.
My beef with them has been about their odd inability to kick ass when handed an opportunity to do so. They enabled MMS long after Apple supported it, they're just now talking about enabling tethering after a couple years of mumbling their intent.
On top of their apparent reticence for superiority, they're not exactly brimming over with competence, either. Their web site has apparently melted down, presumably under a deluge of people trying to order new iphones.
-harry