Just my personal experience...
I have been a Nextel-Sprint customer since 2006, and I have always had pretty good service.
Working at the company I currently work for in NoVA, from 2009 until about 2011, I had a company owned Blackberry on AT&T, and the service was decent at first.
However, around the time that the company went to BYOD, in 2011, I noticed that both the data and voice services seemed to get worse and worse. Since the BYOD policy, I now use my personal device, on the Sprint network, and have no problems.
Also, my other recent experience with AT&T is that my boss has an iPhone on AT&T. When she works remote from her home in Maryland, frequently it is really difficult to hear her or understand her, because her phone keeps cutting-in-and-out. Also sometimes her phone drops calls altogether.
My point is that in my personal experience, it seems to me that while AT&T had a fairly good network in the DC/VA/MD area, the network has degraded, and AT&T has not really done anything to increase capacity or performance.