Pros
Working at a world class agency is definitely a showstopper on your resume and is a great move for your career. The opportunities you have to network with really smart people, work on Fortune 500 accounts, and get experience doing all types of marketing is unparalleled. The people here are also very kind and genuinely care about one another. They have good benefits (including a very generous maternity leave and an annual fitness stipend). Overall, the people are what make it great. It's a very supportive environment...
Cons
...but the cost is low compensation and a bad work/life balance. The pay is not competitive compared to other marketing jobs. When I started, associates made $55K and then move up to $75K if you become senior. Managers make $95K. This is hard if you live in an expensive city. Also, clear your calendar - you will be expected to work nights, weekends, and holidays. It was stressful to log off earlier than 6 or 7 because the teams I was on often worked late. It is also client facing so your deadlines are driven by external forces and often are mandatory which puts a ton of pressure on teams to sacrifice personal time to get the job done. Additionally, the path to promotions is unclear. Promotions take place twice a year and sometimes the company freezes promotions which means no one gets moved up, thus creating a backlog.