Pros
A-list clients who are willing to think big and spend big (as much as anyone will in this economic environment). Really smart, hard-working people and a collegial environment where people help each other out and work as a team. You can find industry-leading experts in almost any aspect of PR here and tap their expertise. This is a great place to learn PR from a big-agency perspective - account management, client services and a wide range of PR strategies and tactics. Management appreciates how hard we work and allows for a lot of latitude in when, where and how people work. They are open and willing to let people switch groups and accounts in order to expand their skill set, in my experience.
Cons
The hours are long and the clients extremely demanding -- more than at the smaller agencies I've worked at. There is a certain amount of office politics at the senior levels and that can trickle down to the rest of the staff. Much of the media relations work is done by media specialists and on some accounts that means entry-level staff don't get much direct experience pitching reporters; candidates who are looking for media relations experience should ask specific questions about this when interviewing.