Pros
They recognize performers compensate accordingly (note however that comp doesn't get much above market until you have 5-7 years of experience). Decent benefits package. Overall a collegial and friendly culture. Local and regional management are accessible and willing to listen. Good support for learning and development. Mercer in one of the top firms in its field and therefore you get lots of great experience and learning opportunities.
Kontras
Big company rules make the firm quite inflexible. Onerous processes and some archaic systems reduce efficiency. Significant changes in senior management team and organizational structure every 2-3 years, which also reduces efficiency since everyone has to adapt. High expectations of employees with regard to work load and output (and getting higher with new management team). No formal orientation/onboarding traning.