Pros
Mercer has a lot of different groups, so my comments might be specific to just the Talent/Consulting arm. The staffing model works like this - you're on multiple projects/teams at one time. If you don't know how to juggle this type of schedule and to deal with managers who are competing with you're time, try a more traditionally staffed job. This is a really great source of development on a 1. project management basis 2. networking basis 3. knowledge basis across various industries/projects/working styles That said, there is ridiculously high turnover at the one-year mark, and very few people who have 'grown up' at the firm and carry the knowledge needed to really get things done well. It's a problem that's compounding upon itself and resulting in the feeling that this is a sinking boat that will sink fast one day when senior people realize they don't have enough junior people to do the work they sell - but instead, have external hires who don't know the Mercer methodology.
Cons
Hours and work/life balance. It's a roll of the dice in terms of technical skill development - no set curriculum, depends on who you are working with and the project type (e.g. six months in, myself and the other new analysts had extremely different skillsets. Be mindful of trying to build up a generalist knowledge base first)