Pros
If you are tough enough to stick it out, and if you are truly passionate about your work, you will gain a lot of valuable experience here. There is a large roster of clients and a lot of different characters (good and bad) which will also add to your experience. This is a great place to start an advertising career, and for the most part if you can make it here you can make it pretty much anywhere.
Cons
Hard work and brains don't always get recognition in this large, often impersonal work environment. You have to make a lot of noise and really put yourself out there to be acknowledged. It's not for everyone. It can sometimes be a sweat shop, with more focus on meeting deadlines and billing hours than quality of work. There is a lot of staff turnover and a great deal of anguish to long-time employees who constantly have to pick up the slack.