Pros
It's a great agency with a lot of smart and passionate people and some big clients to work on. If you work with the right team you can learn a lot.
Cons
There is a clique that runs the agency- the executives have their favorites who will promote people with lacking talent to senior and executive positions and always back them up whatever they do. For those people this company is great- but if you can't become part of the "club" you cannot grow a career there. You'll be stuck with partners backing up their "favorites" if you ever disagree with them. And most of the people they've promoted since the merger (the first one) to exec. roles are people in maybe their early thirties and are not experts in their fields or in managing others, but care more about their perception and power than what is actually right for a client. There's a fostering of learning for people new to the agency and if you can get friendly with the higher-ups then for sure stay there because you're set for life and will probably become a VP long before you deserve it. If you don't get in with them then you need to look for other opportunities ASAP.