Pros
the Nielsen name on your resume, which isn't even worth very much any more. Your power point and excel grunt-work skills will be great after a few months here. By then you will also have more than enough material to write your own dilbert comic strip series.
Cons
Extremely cutthroat environment. rampant scapegoating, rumors, lots of people throwing each other under the bus, and trying to escape the blame because most of the products have chronic issues and the data is always screwed up (business units like BASES and OCR have better product & data quality but culture issues still remain). It doesn't help that people here also take themselves and their jobs way too seriously. Clients often return data; even the non-research/data clients find glaring mistakes on a regular basis. Several syndicated products are in such poor shape that every single year they have a team of black belts come in, spend a week putting up post-it notes and reformulating all their processes, but inevitably nothing helps and the exact same thing happens next year. The roots of this culture go back to the management history; after the company was saddled up with debt and sold to private equity, they wanted to do anything they could to cut costs, recoup their money, and have a successful IPO. During that period before the IPO, which lasted several years, clearly managements priority was milking the business enough to have a strong IPO, not running or growing it in a way that was any way sustainable. Those same management practices have continued. The only 2 circumstances I would recommend working for this company - 1) if you are a huge jerk you will fit right in, or 2) if you are a fresh college grad who has gotten into the Emerging Leaders Program, AND don't have a better offer elsewhere. That rotational program actually provides quite a bit of exposure to different businesses and can be a good experience.