Pros
Some very, very talented people at every level; organized, buttoned up organization that has (for the most part) a clear plan for growth.
Cons
If you are looking to create inspiring, ground-breaking, even close to "cutting edge" experiences, you will starve. The bottom line is the bottom line. The smartest people in the room will (and do) always have the corner office. If you're junior to mid level, this is a hard place navigate--especially when you're ideas will likely never be heard or taken seriously. If you do come from a top level position at a globally recognized agency/brand: you're probably still going to have to fight to maintain that space. I've seen people many years my senior treated like amateurs. I left with very little of substance in my portfolio and an overall feeling that I spent a few years being underutilized and waisted. On the other hand: I do know now what it's like to work for a company going through an acquisition.