Pros
Mercer pays very well. They also have an annual bonus plan that can be 20% (Senior Associate)of your annual salary. They also have a pension plan. It really depends on your role but many roles offer very flexible work arangements include telecommuting. You should never really get bored at Mercer because there is plenty of opportunity to apply and move into other positions during your time at Mercer.
Cons
Very similar to a law firm. The rich keep getting richer. Many of the managers dislike each other so much it's obvious. However, for some reason, they all keep scratching each others back I guess to protect the heard and prevent any imposters from invading their territory. The culture is not a very happy and social one. Egos rule and it's almost hilarious at times. Everyone sees it so it comes down to "whatever, just leave me alone and keep paying me well" which doesn't breed ambition or innovation.