Pros
Working for Kantar gave me the flexibility to continue my Masters Degree whilst also having the time to further my writing career (the long train journey into their westgate office gave me hours of writing time!) in the form of a job that could be broken down into just 3 1/2 hour shifts a day if needed. The supervisors were generally friendly and the place felt fairly relaxed.
Cons
There's a great amount of autonomy with this job; there aren't that many surveys being done, so you're usually saying the same thing over and over for months on end. It's fairly easy to maintain a good work/life balance if you can look at your day in a different way. I can't say anything about promotions as I never went for/got one - I did apply for a Graduate scheme but wasn't even invited for an interview - so I'll leave that there. Time off was easy enough, but you don't get paid!