AT&T Coverage varies a lot from area to area - some areas AT&T is great - others, not so much. One thing I would add, is get current information about how the AT&T service is in your area - If you knew someone who had bad coverage with AT&T a year ago, things may have changed. We got our iPhones last fall, and the coverage around our home was spotty. It was OK at home, but if we headed even a few blocks north to the area we do most of our shopping, the coverage was awful. However over the past few months coverage has improved dramatically. 3G service covers a much larger area and calls no longer drop where they used to drop all the time.
AT&T knows they have a problem with their network - they are bringing on customers like mad because of the iPhone but they know that they risk losing many of them because of their network issues - so AT&T is working to improve coverage etc. The question is, are they improving it fast enough and most importantly for you, have they improved it in the areas where you need it most.