Pros
The people at Mercer are pleasant and know what they're doing, which is refreshing. They're consistently helpful, and most people are social with coworkers. The organization is flatter than others I've worked in - principals have a good deal of interaction with analysts, particularly on small projects where no associates or senior associates are staffed. It wouldn't be right to say that there aren't egos, but they are relatively few; most people are friendly, and those that aren't are penetrable if you enter a meeting with them prepared for their intolerance. Overall, Mercer is a very fair organization and more of a meritocracy than other companies for whom I've worked.
Cons
HR consulting is a narrow field, and the skills acquired in whatever line of business you enter are not particularly transferable. If you're an experienced HR consultant, this is no problem, but if you're a college grad deciding between entry level at Mercer versus entry level at, say, a management consulting firm, this is something to consider. (With that said, actuarial skills are quite transferable, and those I've seen apply to business and law school from non-actuarial roles have fared quite well.) Mercer is also a huge organization, and sometimes the consequences for the company of some other line of business or office's failings will come down on your group. While the Mercer's personality is that of a small firm, it still comes with the other drawbacks of a large company.