Pros
Mercer has a good brand name for future possible movement to the client side of the business. Mercer offers some really intelligent people that you can work with. It offers decent pay compared to some other industries and some internal roles. You get to work with a variety of different clients across a large number of different industries. Mercer tries to staff locally and therefore travel is not too taxing at all. When you do need to travel, they try to keep travel local (so if you're based in the NY office, usually you'll be spending time in the North East. It is good as a first job out of college to understand what is out there in the big world of HR.
Cons
Mercer has a lot of red tape to get anything done. They know what they are good at and they work to churn that out. At times you can feel like a billable widget, churning out billable hours doing the bread and butter work, which undoubtedly may make good business sense since there is a good amount of money that can come from the work but at times can be boring. They talk a good game about expanding businesses, moving into new areas and working cross segment but you don't really see much actually happening. Many initiatives get started, but few are seen through.