Pros
The wireless industry is an exciting, rapidly evolving one. Competition and steady growth bring new challenges and opportunities for continued growth and development. Being part of a company with the scale and history of what is now AT&T brings some advantages in terms of overall corporate stability, employee resources, support for diversity, etc. Much of the work here particularly in the Marketing and Technology units is meaningful and rewarding (though certainly not all) in terms of being able to drive significant revenue, create cutting edge products and services and touch tens of millions of customers. The Mobility unit specifically is the lifeblood of AT&T at the moment -- the growth engine providing fuel for investment in new wireline, broadband and entertainment services while the company continues to manage the decline in traditional wireline access.
Cons
Unfortunately, many of us who have been "here" (in the Redmond/Seattle area specificall) for a fairly long time (10+ years) are nostalgic for the good ol' days of AT&T Wireless when the leadership was local, the company was growing like wildfire and the culture was one that focused much more on employee satisfaction than it is today. Particularly now that the new AT&T (post mergers/acquisitions between CIngular/AT&T Wireless, SBC, Bell South and legacy AT&T) is a company with over 300K employees, the level of bureaucracy has skyrocketed. While many departments still try desparately to move at rapid speed, corporate processes slow things down. Expense management frequently seems to trump all other objectives. Benefits -- once highly competitive -- have sunk to mearly middle of the road. Employee morale and corporate culture varies considerably depending on department and location. A Marketing Mgr in Atlanta could have a very different experience from a Customer Operations Specialist in Bothell or a Technology Architect in Redmond. All would likely agree though that while the company is on balance a good place to work, it's no where near as fun as it used to be.