Pros
I've worked at Mercer for 18 years and had three completely different careers in that time. It's a great place to explore new opportunities, if you push for it. I got my MBA in the evenings while in a consulting role here, which isn't easy, but my colleagues supported me 100%. Everyone is laser-focused on clients, which can make it difficult to get internal initiatives completed but at least there is no question about priorities.
Cons
Mercer is a large, traditional organization. Leaders are trying hard right now to stay relevant in the changing business world, but there is a lot of resistance to change within the organization, so things move very slowly and with great difficulty. Also, pay increases and bonuses over the past several years have been pitiful and discouraging.