Long history, lots of changes
Pros
Strength of the company is from the old Southwestern Bell roots, not from the legacy AT&T company, which was bought by SBC in 2005. SBC took the AT&T name, but the management team was from SBC. Lots of opportunities for development if you are in the right city. Innovative company which has survived throughout the turbulence of the telecom industry. Very talented people working on new technology. Good (but not great) pay and benefits.
Cons
If you are not mobile, you won't advance, regardless of how good your skills are. Many of the people who came over when SBC bought AT&T still have the legacy AT&T mindset of "budget cutting is the only way to be profitable." Assignments in the older (wireline) side of the business are dead end jobs, but getting out of those jobs into other assignments is tough. Outside technical and customer-facing jobs are high pressure, poorly paid, and typically dead-end.