Pros
I was at BASES, which is a smaller unit of Nielsen focused on CPG new product consulting. BASES was a very pleasant place to work, my colleagues were kind and smart almost across the board. Work-life balance is decent (I did 45-50 hours a week I imagine). The managers were good for the most part (mine was fantastic). BASES was thought of as a leading section of the Nielsen Company, so I believe it could be a good starting point for work elsewhere in the company (although many people stay at BASES). As my first job, I learned a lot at BASES about generally navigating the workplace, and was able to find work in another industry based on my experience there.
Cons
New grads should consider that the work BASES does is very industry specific. You will learn a ton about a very narrow set of issues. I think this is a great first job, but if you decide market research isn't for you, leave before you become too specialized. Many people stay at BASES for years. Another thing - the work the company does is becoming more related to consulting, but client contact is quite limited. Also, I don't believe this is a good pre-MBA job - a few people had an MBA or got one through part time programs, but it was uncommon.