Pros
Good people, who share knowledge and work together. Many of the people I have worked with in the past had very strong skill sets, both interpersonal and technical. This definitely assisted me in my career development. Plus you can get nice exposure to other areas of the company and other geographic locales, since it's such a large company. However, I migh have had a different opinion if I was working in a different area of the country, perhaps in a smaller office. Additionally, the culture is relatively relaxed in terms of when you're expected to be at work. Although there are billable hours requirements, when you bill those hours can be pretty flexible.
Cons
Corporate drudgery. There are a lot of people doing what you're doing in any given position. If you are doing something original, it probably won't be good enough to move up if you're not bringing in clients. Additionally, you may end up working extremely hard at Mercer for not that much compensation. Many times I found myself saying that if they just paid me more I'd be satisfied working there, since the work was challenging and intellectually stimulating. However, intellectual stimulation does not pay the bills. There is also too much bureaucracy in how things are accomplished at Mercer. There are many requirements that are in place for no other reason than to cover an executive's bad decision.