Pros
I was only an intern with Mercer, so I cannot comment on all aspects of professional development, but I was extremely satisfied with my experience there. I came in with little knowledge of what the company did, but left feeling confident in my experience and abilities. I was charged with handling complex projects and worked with a wide variety of people. At the end of the summer they have all interns complete an internal presentation, fielding questions as if they were presenting to actual clients. Another huge perk was the constant free food and happy hours! Although they were in a hiring freeze at the end of the summer, they did come back and offer me full-time employment a few months later.
Cons
Because the company is so large, there are a lot of required training sessions and town hall that aren't necessarily relevant to all. But not a huge downer.