Pros
AT&T is a stable company with one of the world's best known "brands". Everyone knows who AT&T is. They are committed to ethical business practices which is why they avoided the problems that Worldcom/MCI had. AT&T is also genuine in their support of diversity regardless of race/religion/orientation.
Cons
Because of their size, AT&T is not at all flexible, either with their employees, their customers or their way of doing business. Even with the size of the company, there are limited career growth opportunities. Most departments are very understaffed for current work loads with plans for further cuts. Salary/compensation packages are 10%-15% below comparable positions at Verizon or Qwest. After several mergers SBC/AT&T/BellSouth/Cingular and others, there is little integration between the various companies, each retaining their own policies and proceedures, meaning that its difficult to do business as "one company". Training opportunities are limited, most are web based and are of poor quality.