Pros
Merkle goes through a thoughtful process to hire people. As a result, they have really smart people who know their stuff. If you are early career, it is a great place to learn. Also, because they have such big clients, you earn lots of "street cred" as you build your resume. When you start interviewing for your next role, you get to brag about working with some of the largest brands in the world.
Cons
The biggest con is that they are leaning on offshore talent a ton! When I left, they were starting to offshore so much that it was turning into a production heavy as opposed to strategy heavy agency. It is no longer the amazing company that David Williams, Craig Dempster and others had build.