Pros
The management and people are all great, low pressure, no micro-managing, a steady flow of work with my client so I'm always busy. They will cater to your interests and try to help you develop a career path to reach your goals. I might be lucky, my direct manager is probably one of the best in the building, if not the business, but he leads by example not fear or demands. He's extremely hard working and you want to do well and work hard to impress him, not because you're afraid of being scolded for errors.
Cons
I have not been here very long, but my co-workers who are the best at their jobs have been here for a while without ever receiving a raise or promotion, while many of the more senior analysts are some of the worst at their jobs. I'm not sure if this is specific to Mercer or just an issue with corporate jobs in general, where experience is valued over raw skill, where some dimwit who's been in the field for 6 years gets better pay than truly talented individuals who have been in it for 2.