Vantaggi
I worked at Kantar Millward Brown, but this can apply to most of Kantar - the company cares about it's employees. I've seen people moved to new operating companies within Kantar instead of letting them go because of the quality of work they do (even when the market needs change). Leadership is transparent about the health of the company (consistent slow growth), and where it's going. Salary was competitive, and when I left I was given a generous counter-offer. Great training, some of the worlds best-known brands, and a high-level of research acumen. Had 2 amazing managers, who made the company even better. There was other mgmt who was less in-touch with juniors, but that translated to lesser professional development not micromanaging or terrible boss status.
Svantaggi
Long hours, but that's not uncommon in market research, consulting, or supplier side in-general. The company is fairly compartmentalized, so you'll become very good at a handful of things, but won't have the opportunity to really try something new (my reason for leaving).