Vantaggi
Lots of opportunities to learn, possibility of travelling or living abroad, can get experience in different types of roles, opportunity to work on large scale projects, the hours are long but not impossible, Christmas parties are epic. If you are willing to work very hard there are good career opportunities.
Svantaggi
Unless you are far up in the company hierarchy don't expect to get any credit for all your hard work - somebody else will, the era of being encouraged to "question anything" is over (previously this was ingrained in the company's ethos), there is a limited range of clients, not many small fun projects to break things up, pay isn't as high as other agencies, could be more focus on creativity and some managers need to listen more.