Pros
Mercer has great brand recognition for their industry and are a very professional company. The client-facing professionals are top-notch and know their business. The benefits are good and there are some opportunities to relocate if need be. Your teammates and department will greatly determine the tone of your employment at Mercer.
Cons
While Mercer does a good job of promoting and tracking their consultants from analyst to associate to principal etc, the roles of the internal functions are surprisingly underconsidered. Internal HR is a mess, with almost no local points of contact and abyssmal follow-up. Career tracking is essentially non-existent for internal roles - nearly all internal functions are external hires. And while theoretically Mercer hires laterally, in the words of two supervisors, "I've never seen it happen." The irony of being an HR consulting firm with critical HR issues has recently been addressed by including some of the consultants in discussions regarding internal HR; the shocked disbelief of consultants returning from these management meetings speaks for itself.