Pros
The only pro I can think of: I learned quite a bit while I was at Mercer, largely because they throw you in the deep end and never really let up -- ever.
Cons
The culture of my practice area was completely toxic. The senior leaders of the group pushed consultants constantly, showed virtually no support or appreciation, and criticized consultants' work relentlessly. It can drive a certain type to achieve and surpass -- but then comes burnout. Comp isn't great either, considerably lower than at a direct competitor firm, so there's no real reward. The group I worked in has seen huge, cyclical turnover for years, which should tell you something. I'm baffled...doesn't anyone crunch the bottom-line numbers of recruitment & hiring costs against revenue? (Not to mention the hidden costs of losing experience and IC, and forever having to train/ramp-up new hires.)