Pros
You get exposed to a lot of companies and you learn a lot about employer benefits. There's a kind of unwarranted, self-proclaimed reputation that Mercer has among benefits consulting firms. Don't be fooled.
Kontras
Many senior consultants here are probably pathologically psychotic. This is a problem insofar as you're subject to their psychosis. To be honest, it may have been the result of the fraternity-like hazing they had to endure in the riveting world of benefits to get to where they are. Being successful at Mercer, particularly in their health and benefits division, has a lot to do with being able to cope with psychosis (i.e. getting reamed, meeting absurd expectations, compromising on ethical values) - this is often called "managing personalities".