Pros
AT&T is a conglomerate that you want to work for. They have very competitive salaries and the work environment is very relaxed yet perfessional. Although there are a lot of employees that probably could be let go and tend to just do enough to get by, AT&T also has some of the top talent in the industry and those individuals quickly rise to the top. AT&T is a process driven corporation and is built around service delivery in each department. There is little scope creep yet under guidance you are allowed to pursue special projects that can get you noticed for favorable treatment or escalation.
Cons
The cons are the same in all big business, there are a lot of individuals that do not have any desire to perform and there seemed to be little to be done by mid-management to motivate them or move them out. Also, some of the leadership could be a little presumptuous and there was little room for autonomy and innovation. Lastly, it can be very difficult to move from one organization to another due to HR politics.