Pros
Some of the brightest people in the actuarial world work there - being able to talk with senior consultants who won't "catch a fish for you" but will help you "learn how to fish for yourself" means your development is constant. During my tenure, there were lots of younger people working there and that made for some life-long friendships. Exciting to meet up with them again on the client-side, as many have pursued opportunities beyond Mercer's borders, especially here in Calgary.
Kontras
The bulk of the work skews to the lower end of the pay structure, which helps perpetuate the notion that once you've made it, you get to handle the interesting clients and make the newbies do all the crap work. But hey, that's how I learned to be really good at what I do (competence + confidence), to be thoughtful to folks from all levels of an organization (respectful) and to become a great manager of people in future chapters (patience + coaching).