Pros
The culture. The culture creates an environment where "the cream rises to the top." It is also a culture where when asked the leaders will take the time to explain to you what you need to do to reach the level above where you are. They actively encourage and reward their employees for their community service. I've worked places where you can't mention having any outside commitments because that would be seen as detracting from the time you should be spending on career but RSM has programs to award employees for their community service and there are many many people in the company that are active in their community.
Cons
The only downside to working at RSM is just the aches and pains of an organization that is growing and has a great number of employees in total. Not many people have heard of RSM and that can be a definite downside when it comes to people asking where you work. I often have to explain to people where I work since they haven't heard of it. It really doesn't have a strong downside and I've worked enough other places so that when I think of this question I can readily site some really bad things about the culture, environment or management of a few of my previous employers but I just don't have those stories about RSM.